diff --git a/Changes b/Changes
index ec20c6fda..bfbc7dc92 100644
--- a/Changes
+++ b/Changes
@@ -17,6 +17,7 @@ Verilator 4.223 devel
 
 **Minor:**
 
+* Add -f<optimization> options to replace -O<letter> options (#3436).
 * Changed --no-merge-const-pool to -fno-merge-const-pool (#3436).
 * Support compile time trace signal selection with tracing_on/off (#3323). [Shunyao CAD]
 * Add assert when VerilatedContext is mis-deleted (#3121). [Rupert Swarbrick]
diff --git a/bin/verilator b/bin/verilator
index 367651d32..f04e2a593 100755
--- a/bin/verilator
+++ b/bin/verilator
@@ -319,7 +319,7 @@ detailed descriptions of these arguments.
      -f <file>                  Parse arguments from a file
      -FI <file>                 Force include of a file
     --flatten                   Force inlining of all modules, tasks and functions
-    --fno-merge-const-pool      Disable merging of different types in const pool
+    --fno-<optimization>        Disable internal optimization stage
      -G<name>=<value>           Overwrite top-level parameter
     --gdb                       Run Verilator under GDB interactively
     --gdbbt                     Run Verilator under GDB for backtrace
diff --git a/docs/guide/deprecations.rst b/docs/guide/deprecations.rst
index 33c2ef610..4c2d96592 100644
--- a/docs/guide/deprecations.rst
+++ b/docs/guide/deprecations.rst
@@ -20,6 +20,11 @@ Option `--cdc`
   The experimental `--cdc` option is believed to be generally unused and is
   planned for removal no sooner than January 2023.
 
+Option `--O<letter>`
+  The debug `--O<letter>` options have been replaced with
+  `--fno-<optimization>` debug options to match GCC. The old options are
+  planned for removal no sooner than June 2023.
+
 Option `--prof-threads`
   The `--prof-threads` option has been superseded by the `--prof-exec` and
   `--prof-pgo` options and is planned for removal no sooner than April 2023.
diff --git a/docs/guide/exe_verilator.rst b/docs/guide/exe_verilator.rst
index 70b3752ad..fda5aedb3 100644
--- a/docs/guide/exe_verilator.rst
+++ b/docs/guide/exe_verilator.rst
@@ -431,13 +431,51 @@ Summary:
    flattening large designs may require significant CPU time, memory and
    storage.
 
+.. option:: --fno-acyc-simp
+
+.. option:: --fno-assemble
+
+.. option:: --fno-case
+
+.. option:: --fno-combine
+
+.. option:: --fno-const
+
+.. option:: --fno-const-bit-op-tree
+
+.. option:: --fno-dedup
+
+.. option:: --fno-expand
+
+.. option:: --fno-gate
+
+.. option:: --fno-inline
+
+.. option:: --fno-life
+
+.. option:: --fno-life-post
+
+.. option:: --fno-localize
+
+.. option:: --fno-merge-cond
+
 .. option:: --fno-merge-const-pool
 
-   Rarely needed; only use if recommended by maintainers.  In order to
-   minimize cache footprint, values of different data type, that are yet
-   emitted identically in C++ are merged in the constant pool.  This option
-   disables this and causes every constant pool entry with a distinct data
-   type to be emitted separately.
+.. option:: --fno-reloop
+
+.. option:: --fno-reorder
+
+.. option:: --fno-split
+
+.. option:: --fno-subst
+
+.. option:: --fno-subst-const
+
+.. option:: --fno-table
+
+   Rarely needed. Disables one of the internal optimization steps. These
+   are typically used only when recommended by a maintainer to help debug
+   or work around an issue.
 
 .. option:: -G<name>=<value>
 
@@ -704,9 +742,9 @@ Summary:
 
    Rarely needed.  Enables or disables a specific optimizations, with the
    optimization selected based on the letter passed.  A lowercase letter
-   disables an optimization, an upper case letter enables it.  This is
-   intended for debugging use only; see the source code for
-   version-dependent mappings of optimizations to -O letters.
+   disables an optimization, an upper case letter enables it.  This option
+   is deprecated and the various `-f<optimization>` arguments should be
+   used instead.
 
 .. option:: -o <executable>
 
diff --git a/src/V3Case.cpp b/src/V3Case.cpp
index 161f7db7e..c65fb3e7d 100644
--- a/src/V3Case.cpp
+++ b/src/V3Case.cpp
@@ -496,7 +496,7 @@ private:
         V3Case::caseLint(nodep);
         iterateChildren(nodep);
         if (debug() >= 9) nodep->dumpTree(cout, " case_old: ");
-        if (isCaseTreeFast(nodep) && v3Global.opt.oCase()) {
+        if (isCaseTreeFast(nodep) && v3Global.opt.fCase()) {
             // It's a simple priority encoder or complete statement
             // we can make a tree of statements to avoid extra comparisons
             ++m_statCaseFast;
diff --git a/src/V3Const.cpp b/src/V3Const.cpp
index 9f39e2884..e246180a2 100644
--- a/src/V3Const.cpp
+++ b/src/V3Const.cpp
@@ -1090,7 +1090,7 @@ private:
 
     bool matchBitOpTree(AstNode* nodep) {
         if (nodep->widthMin() != 1) return false;
-        if (!v3Global.opt.oConstBitOpTree()) return false;
+        if (!v3Global.opt.fConstBitOpTree()) return false;
 
         string debugPrefix;
         if (debug() >= 9) {  // LCOV_EXCL_START
@@ -1412,7 +1412,7 @@ private:
         return (VN_IS(nodep, And) || VN_IS(nodep, Or) || VN_IS(nodep, Xor));
     }
     bool ifAdjacentSel(const AstSel* lhsp, const AstSel* rhsp) {
-        if (!v3Global.opt.oAssemble()) return false;  // opt disabled
+        if (!v3Global.opt.fAssemble()) return false;  // opt disabled
         if (!lhsp || !rhsp) return false;
         const AstNode* const lfromp = lhsp->fromp();
         const AstNode* const rfromp = rhsp->fromp();
@@ -1427,7 +1427,7 @@ private:
     }
     bool ifMergeAdjacent(AstNode* lhsp, AstNode* rhsp) {
         // called by concatmergeable to determine if {lhsp, rhsp} make sense
-        if (!v3Global.opt.oAssemble()) return false;  // opt disabled
+        if (!v3Global.opt.fAssemble()) return false;  // opt disabled
         // two same varref
         if (operandsSame(lhsp, rhsp)) return true;
         const AstSel* lselp = VN_CAST(lhsp, Sel);
@@ -1464,7 +1464,7 @@ private:
     }
     bool concatMergeable(const AstNode* lhsp, const AstNode* rhsp, unsigned depth) {
         // determine if {a OP b, c OP d} => {a, c} OP {b, d} is advantageous
-        if (!v3Global.opt.oAssemble()) return false;  // opt disabled
+        if (!v3Global.opt.fAssemble()) return false;  // opt disabled
         if (lhsp->type() != rhsp->type()) return false;
         if (!ifConcatMergeableBiop(lhsp)) return false;
         if (depth > CONCAT_MERGABLE_MAX_DEPTH) return false;  // As worse case O(n^2) algorithm
@@ -2550,7 +2550,7 @@ private:
             if (nodep->access().isReadOnly()
                 && ((!m_params  // Can reduce constant wires into equations
                      && m_doNConst
-                     && v3Global.opt.oConst()
+                     && v3Global.opt.fConst()
                      // Default value, not a "known" constant for this usage
                      && !nodep->varp()->isClassMember()
                      && !(nodep->varp()->isFuncLocal() && nodep->varp()->isNonOutput())
diff --git a/src/V3Gate.cpp b/src/V3Gate.cpp
index 4b66c2661..cf3485121 100644
--- a/src/V3Gate.cpp
+++ b/src/V3Gate.cpp
@@ -397,11 +397,11 @@ private:
         // Then propagate more complicated equations
         optimizeSignals(true);
         // Remove redundant logic
-        if (v3Global.opt.oDedupe()) {
+        if (v3Global.opt.fDedupe()) {
             dedupe();
             if (debug() >= 6) m_graph.dumpDotFilePrefixed("gate_dedup");
         }
-        if (v3Global.opt.oAssemble()) {
+        if (v3Global.opt.fAssemble()) {
             mergeAssigns();
             if (debug() >= 6) m_graph.dumpDotFilePrefixed("gate_assm");
         }
diff --git a/src/V3GraphAcyc.cpp b/src/V3GraphAcyc.cpp
index a62fd3d9d..0df758ed1 100644
--- a/src/V3GraphAcyc.cpp
+++ b/src/V3GraphAcyc.cpp
@@ -254,7 +254,7 @@ void GraphAcyc::simplify(bool allowCut) {
         if (allowCut) {
             // The main algorithm works without these, though slower
             // So if changing the main algorithm, comment these out for a test run
-            if (v3Global.opt.oAcycSimp()) {
+            if (v3Global.opt.fAcycSimp()) {
                 cutBasic(vertexp);
                 cutBackward(vertexp);
             }
diff --git a/src/V3Options.cpp b/src/V3Options.cpp
index 2a4c3050d..7d4b1e846 100644
--- a/src/V3Options.cpp
+++ b/src/V3Options.cpp
@@ -1082,7 +1082,28 @@ void V3Options::parseOptsList(FileLine* fl, const string& optdir, int argc, char
         parseOptsFile(fl, parseFileArg(optdir, valp), false);
     });
     DECL_OPTION("-flatten", OnOff, &m_flatten);
+
+    DECL_OPTION("-facyc-simp", FOnOff, &m_fAcycSimp);
+    DECL_OPTION("-fassemble", FOnOff, &m_fAssemble);
+    DECL_OPTION("-fcase", FOnOff, &m_fCase);
+    DECL_OPTION("-fcombine", FOnOff, &m_fCombine);
+    DECL_OPTION("-fconst", FOnOff, &m_fConst);
+    DECL_OPTION("-fconst-bit-op-tree", FOnOff, &m_fConstBitOpTree);
+    DECL_OPTION("-fdedup", FOnOff, &m_fDedupe);
+    DECL_OPTION("-fexpand", FOnOff, &m_fExpand);
+    DECL_OPTION("-fgate", FOnOff, &m_fGate);
+    DECL_OPTION("-finline", FOnOff, &m_fInline);
+    DECL_OPTION("-flife", FOnOff, &m_fLife);
+    DECL_OPTION("-flife-post", FOnOff, &m_fLifePost);
+    DECL_OPTION("-flocalize", FOnOff, &m_fLocalize);
+    DECL_OPTION("-fmerge-cond", FOnOff, &m_fMergeCond);
     DECL_OPTION("-fmerge-const-pool", FOnOff, &m_fMergeConstPool);
+    DECL_OPTION("-freloop", FOnOff, &m_fReloop);
+    DECL_OPTION("-freorder", FOnOff, &m_fReorder);
+    DECL_OPTION("-fsplit", FOnOff, &m_fSplit);
+    DECL_OPTION("-fsubst", FOnOff, &m_fSubst);
+    DECL_OPTION("-fsubst-const", FOnOff, &m_fSubstConst);
+    DECL_OPTION("-ftable", FOnOff, &m_fTable);
 
     DECL_OPTION("-G", CbPartialMatch, [this](const char* optp) { addParameter(optp, false); });
     DECL_OPTION("-gate-stmts", Set, &m_gateStmts);
@@ -1155,47 +1176,49 @@ void V3Options::parseOptsList(FileLine* fl, const string& optdir, int argc, char
     DECL_OPTION("-max-num-width", Set, &m_maxNumWidth);
     DECL_OPTION("-mod-prefix", Set, &m_modPrefix);
 
-    DECL_OPTION("-O", CbPartialMatch, [this](const char* optp) {
-        // Optimization
+    DECL_OPTION("-O0", CbCall, [this]() { optimize(0); });
+    DECL_OPTION("-O1", CbCall, [this]() { optimize(1); });
+    DECL_OPTION("-O2", CbCall, [this]() { optimize(2); });
+    DECL_OPTION("-O3", CbCall, [this]() { optimize(3); });
+
+    DECL_OPTION("-O", CbPartialMatch, [this, fl](const char* optp) {
+        // Optimization, e.g. -O1rX
+        // LCOV_EXCL_START
+        fl->v3warn(DEPRECATED, "Option -O<letter> is deprecated. "
+                               "Use -f<optimization> or -fno-<optimization> instead.");
         for (const char* cp = optp; *cp; ++cp) {
             const bool flag = isupper(*cp);
             switch (tolower(*cp)) {
-            case '0': optimize(0); break;  // 0=all off
-            case '1': optimize(1); break;  // 1=all on
-            case '2': optimize(2); break;  // 2=not used
-            case '3': optimize(3); break;  // 3=high
-            case 'a': m_oTable = flag; break;
-            case 'b': m_oCombine = flag; break;
-            case 'c': m_oConst = flag; break;
-            case 'd': m_oDedupe = flag; break;
-            case 'e': m_oCase = flag; break;
-            //    f
-            case 'g': m_oGate = flag; break;
-            //    h
-            case 'i': m_oInline = flag; break;
-            //    j
-            case 'k': m_oSubstConst = flag; break;
-            case 'l': m_oLife = flag; break;
-            case 'm': m_oAssemble = flag; break;
-            //    n
-            case 'o':
-                m_oConstBitOpTree = flag;
-                break;  // Can remove ~2022-01 when stable
-            //    o will be used as an escape for a second character of optimization disables
+            case '0': optimize(0); break;
+            case '1': optimize(1); break;
+            case '2': optimize(2); break;
+            case '3': optimize(3); break;
+            case 'a': m_fTable = flag; break;  // == -fno-table
+            case 'b': m_fCombine = flag; break;  // == -fno-combine
+            case 'c': m_fConst = flag; break;  // == -fno-const
+            case 'd': m_fDedupe = flag; break;  // == -fno-dedup
+            case 'e': m_fCase = flag; break;  // == -fno-case
+            case 'g': m_fGate = flag; break;  // == -fno-gate
+            case 'i': m_fInline = flag; break;  // == -fno-inline
+            case 'k': m_fSubstConst = flag; break;  // == -fno-subst-const
+            case 'l': m_fLife = flag; break;  // == -fno-life
+            case 'm': m_fAssemble = flag; break;  // == -fno-assemble
+            case 'o': m_fConstBitOpTree = flag; break;  // == -fno-const-bit-op-tree
             case 'p':
                 m_public = !flag;
                 break;  // With -Op so flag=0, we want public on so few optimizations done
-            //    q
-            case 'r': m_oReorder = flag; break;
-            case 's': m_oSplit = flag; break;
-            case 't': m_oLifePost = flag; break;
-            case 'u': m_oSubst = flag; break;
-            case 'v': m_oReloop = flag; break;
-            case 'w': m_oMergeCond = flag; break;
-            case 'x': m_oExpand = flag; break;
-            case 'y': m_oAcycSimp = flag; break;
-            case 'z': m_oLocalize = flag; break;
-            default: break;  // No error, just ignore
+            case 'r': m_fReorder = flag; break;  // == -fno-reorder
+            case 's': m_fSplit = flag; break;  // == -fno-split
+            case 't': m_fLifePost = flag; break;  // == -fno-life-post
+            case 'u': m_fSubst = flag; break;  // == -fno-subst
+            case 'v': m_fReloop = flag; break;  // == -fno-reloop
+            case 'w': m_fMergeCond = flag; break;  // == -fno-merge-cond
+            case 'x': m_fExpand = flag; break;  // == -fno-expand
+            case 'y': m_fAcycSimp = flag; break;  // == -fno-acyc-simp
+            case 'z': m_fLocalize = flag; break;  // == -fno-localize
+            default:
+                break;  // No error, just ignore
+                // LCOV_EXCL_STOP
             }
         }
     });
@@ -1787,26 +1810,26 @@ int V3Options::dumpTreeLevel(const string& srcfile_path) {
 void V3Options::optimize(int level) {
     // Set all optimizations to on/off
     const bool flag = level > 0;
-    m_oAcycSimp = flag;
-    m_oAssemble = flag;
-    m_oCase = flag;
-    m_oCombine = flag;
-    m_oConst = flag;
-    m_oConstBitOpTree = flag;
-    m_oDedupe = flag;
-    m_oExpand = flag;
-    m_oGate = flag;
-    m_oInline = flag;
-    m_oLife = flag;
-    m_oLifePost = flag;
-    m_oLocalize = flag;
-    m_oMergeCond = flag;
-    m_oReloop = flag;
-    m_oReorder = flag;
-    m_oSplit = flag;
-    m_oSubst = flag;
-    m_oSubstConst = flag;
-    m_oTable = flag;
+    m_fAcycSimp = flag;
+    m_fAssemble = flag;
+    m_fCase = flag;
+    m_fCombine = flag;
+    m_fConst = flag;
+    m_fConstBitOpTree = flag;
+    m_fDedupe = flag;
+    m_fExpand = flag;
+    m_fGate = flag;
+    m_fInline = flag;
+    m_fLife = flag;
+    m_fLifePost = flag;
+    m_fLocalize = flag;
+    m_fMergeCond = flag;
+    m_fReloop = flag;
+    m_fReorder = flag;
+    m_fSplit = flag;
+    m_fSubst = flag;
+    m_fSubstConst = flag;
+    m_fTable = flag;
     // And set specific optimization levels
     if (level >= 3) {
         m_inlineMult = -1;  // Maximum inlining
diff --git a/src/V3Options.h b/src/V3Options.h
index e1756ab3d..137580c34 100644
--- a/src/V3Options.h
+++ b/src/V3Options.h
@@ -239,7 +239,6 @@ private:
     bool m_dumpDefines = false;     // main switch: --dump-defines
     bool m_dumpTreeAddrids = false; // main switch: --dump-tree-addrids
     bool m_exe = false;             // main switch: --exe
-    bool m_fMergeConstPool = true;  // main switch: --fmerge-const-pool
     bool m_flatten = false;         // main switch: --flatten
     bool m_hierarchical = false;    // main switch: --hierarchical
     bool m_hierChild = false;       // main switch: --hierarchical-child
@@ -341,27 +340,27 @@ private:
     V3LangCode  m_defaultLanguage;      // main switch: --language
 
     // MEMBERS (optimizations)
-    //                          // main switch: -Op: --public
-    bool        m_oAcycSimp;    // main switch: -Oy: acyclic pre-optimizations
-    bool        m_oAssemble;    // main switch: -Om: assign assemble
-    bool        m_oCase;        // main switch: -Oe: case tree conversion
-    bool        m_oCombine;     // main switch: -Ob: common icode packing
-    bool        m_oConst;       // main switch: -Oc: constant folding
-    bool        m_oConstBitOpTree;  // main switch: -Oo: constant bit op tree
-    bool        m_oDedupe;      // main switch: -Od: logic deduplication
-    bool        m_oExpand;      // main switch: -Ox: expansion of C macros
-    bool        m_oGate;        // main switch: -Og: gate wire elimination
-    bool        m_oInline;      // main switch: -Oi: module inlining
-    bool        m_oLife;        // main switch: -Ol: variable lifetime
-    bool        m_oLifePost;    // main switch: -Ot: delayed assignment elimination
-    bool        m_oLocalize;    // main switch: -Oz: convert temps to local variables
-    bool        m_oMergeCond;   // main switch: -Ob: merge conditionals
-    bool        m_oReloop;      // main switch: -Ov: reform loops
-    bool        m_oReorder;     // main switch: -Or: reorder assignments in blocks
-    bool        m_oSplit;       // main switch: -Os: always assignment splitting
-    bool        m_oSubst;       // main switch: -Ou: substitute expression temp values
-    bool        m_oSubstConst;  // main switch: -Ok: final constant substitution
-    bool        m_oTable;       // main switch: -Oa: lookup table creation
+    bool m_fAcycSimp;    // main switch: -fno-acyc-simp: acyclic pre-optimizations
+    bool m_fAssemble;    // main switch: -fno-assemble: assign assemble
+    bool m_fCase;        // main switch: -fno-case: case tree conversion
+    bool m_fCombine;     // main switch: -fno-combine: common icode packing
+    bool m_fConst;       // main switch: -fno-const: constant folding
+    bool m_fConstBitOpTree;  // main switch: -fno-const-bit-op-tree constant bit op tree
+    bool m_fDedupe;      // main switch: -fno-dedupe: logic deduplication
+    bool m_fExpand;      // main switch: -fno-expand: expansion of C macros
+    bool m_fGate;        // main switch: -fno-gate: gate wire elimination
+    bool m_fInline;      // main switch: -fno-inline: module inlining
+    bool m_fLife;        // main switch: -fno-life: variable lifetime
+    bool m_fLifePost;    // main switch: -fno-life-post: delayed assignment elimination
+    bool m_fLocalize;    // main switch: -fno-localize: convert temps to local variables
+    bool m_fMergeCond;   // main switch: -fno-merge-cond: merge conditionals
+    bool m_fMergeConstPool = true;  // main switch: --fmerge-const-pool
+    bool m_fReloop;      // main switch: -fno-reloop: reform loops
+    bool m_fReorder;     // main switch: -fno-reorder: reorder assignments in blocks
+    bool m_fSplit;       // main switch: -fno-split: always assignment splitting
+    bool m_fSubst;       // main switch: -fno-subst: substitute expression temp values
+    bool m_fSubstConst;  // main switch: -fno-subst-const: final constant substitution
+    bool m_fTable;       // main switch: -fno-table: lookup table creation
     // clang-format on
 
     bool m_available = false;  // Set to true at the end of option parsing
@@ -448,7 +447,6 @@ public:
     bool dpiHdrOnly() const { return m_dpiHdrOnly; }
     bool dumpDefines() const { return m_dumpDefines; }
     bool exe() const { return m_exe; }
-    bool fMergeConstPool() const { return m_fMergeConstPool; }
     bool flatten() const { return m_flatten; }
     bool gmake() const { return m_gmake; }
     bool threadsDpiPure() const { return m_threadsDpiPure; }
@@ -575,26 +573,27 @@ public:
     bool isNoClocker(const string& signame) const;
 
     // ACCESSORS (optimization options)
-    bool oAcycSimp() const { return m_oAcycSimp; }
-    bool oAssemble() const { return m_oAssemble; }
-    bool oCase() const { return m_oCase; }
-    bool oCombine() const { return m_oCombine; }
-    bool oConst() const { return m_oConst; }
-    bool oConstBitOpTree() const { return m_oConstBitOpTree; }
-    bool oDedupe() const { return m_oDedupe; }
-    bool oExpand() const { return m_oExpand; }
-    bool oGate() const { return m_oGate; }
-    bool oInline() const { return m_oInline; }
-    bool oLife() const { return m_oLife; }
-    bool oLifePost() const { return m_oLifePost; }
-    bool oLocalize() const { return m_oLocalize; }
-    bool oMergeCond() const { return m_oMergeCond; }
-    bool oReloop() const { return m_oReloop; }
-    bool oReorder() const { return m_oReorder; }
-    bool oSplit() const { return m_oSplit; }
-    bool oSubst() const { return m_oSubst; }
-    bool oSubstConst() const { return m_oSubstConst; }
-    bool oTable() const { return m_oTable; }
+    bool fAcycSimp() const { return m_fAcycSimp; }
+    bool fAssemble() const { return m_fAssemble; }
+    bool fCase() const { return m_fCase; }
+    bool fCombine() const { return m_fCombine; }
+    bool fConst() const { return m_fConst; }
+    bool fConstBitOpTree() const { return m_fConstBitOpTree; }
+    bool fDedupe() const { return m_fDedupe; }
+    bool fExpand() const { return m_fExpand; }
+    bool fGate() const { return m_fGate; }
+    bool fInline() const { return m_fInline; }
+    bool fLife() const { return m_fLife; }
+    bool fLifePost() const { return m_fLifePost; }
+    bool fLocalize() const { return m_fLocalize; }
+    bool fMergeCond() const { return m_fMergeCond; }
+    bool fMergeConstPool() const { return m_fMergeConstPool; }
+    bool fReloop() const { return m_fReloop; }
+    bool fReorder() const { return m_fReorder; }
+    bool fSplit() const { return m_fSplit; }
+    bool fSubst() const { return m_fSubst; }
+    bool fSubstConst() const { return m_fSubstConst; }
+    bool fTable() const { return m_fTable; }
 
     string traceClassBase() const { return m_traceFormat.classBase(); }
     string traceClassLang() const { return m_traceFormat.classBase() + (systemC() ? "Sc" : "C"); }
diff --git a/src/Verilator.cpp b/src/Verilator.cpp
index e233a041c..97e3393c4 100644
--- a/src/Verilator.cpp
+++ b/src/Verilator.cpp
@@ -239,7 +239,7 @@ static void process() {
         // Module inlining
         // Cannot remove dead variables after this, as alias information for final
         // V3Scope's V3LinkDot is in the AstVar.
-        if (v3Global.opt.oInline()) {
+        if (v3Global.opt.fInline()) {
             V3Inline::inlineAll(v3Global.rootp());
             V3LinkDot::linkDotArrayed(v3Global.rootp());  // Cleanup as made new modules
         }
@@ -310,11 +310,11 @@ static void process() {
         // Push constants across variables and remove redundant assignments
         V3Const::constifyAll(v3Global.rootp());
 
-        if (v3Global.opt.oLife()) V3Life::lifeAll(v3Global.rootp());
+        if (v3Global.opt.fLife()) V3Life::lifeAll(v3Global.rootp());
 
         // Make large low-fanin logic blocks into lookup tables
         // This should probably be done much later, once we have common logic elimination.
-        if (!v3Global.opt.lintOnly() && v3Global.opt.oTable()) {
+        if (!v3Global.opt.lintOnly() && v3Global.opt.fTable()) {
             V3Table::tableAll(v3Global.rootp());
         }
 
@@ -328,7 +328,7 @@ static void process() {
         V3Active::activeAll(v3Global.rootp());
 
         // Split single ALWAYS blocks into multiple blocks for better ordering chances
-        if (v3Global.opt.oSplit()) V3Split::splitAlwaysAll(v3Global.rootp());
+        if (v3Global.opt.fSplit()) V3Split::splitAlwaysAll(v3Global.rootp());
         V3SplitAs::splitAsAll(v3Global.rootp());
 
         // Create tracing sample points, before we start eliminating signals
@@ -340,11 +340,11 @@ static void process() {
 
         // Gate-based logic elimination; eliminate signals and push constant across cell boundaries
         // Instant propagation makes lots-o-constant reduction possibilities.
-        if (v3Global.opt.oGate()) {
+        if (v3Global.opt.fGate()) {
             V3Gate::gateAll(v3Global.rootp());
             // V3Gate calls constant propagation itself.
         } else {
-            v3info("Command Line disabled gate optimization with -Og/-O0.  "
+            v3info("Command Line disabled gate optimization with -fno-gate.  "
                    "This may cause ordering problems.");
         }
 
@@ -363,7 +363,7 @@ static void process() {
         }
 
         // Reorder assignments in pipelined blocks
-        if (v3Global.opt.oReorder()) V3Split::splitReorderAll(v3Global.rootp());
+        if (v3Global.opt.fReorder()) V3Split::splitReorderAll(v3Global.rootp());
 
         // Create delayed assignments
         // This creates lots of duplicate ACTIVES so ActiveTop needs to be after this step
@@ -388,11 +388,11 @@ static void process() {
         // Cleanup any dly vars or other temps that are simple assignments
         // Life must be done before Subst, as it assumes each CFunc under
         // _eval is called only once.
-        if (v3Global.opt.oLife()) {
+        if (v3Global.opt.fLife()) {
             V3Const::constifyAll(v3Global.rootp());
             V3Life::lifeAll(v3Global.rootp());
         }
-        if (v3Global.opt.oLifePost()) V3LifePost::lifepostAll(v3Global.rootp());
+        if (v3Global.opt.fLifePost()) V3LifePost::lifepostAll(v3Global.rootp());
 
         // Remove unused vars
         V3Const::constifyAll(v3Global.rootp());
@@ -422,13 +422,13 @@ static void process() {
         v3Global.assertScoped(false);
 
         // Move variables from modules to function local variables where possible
-        if (v3Global.opt.oLocalize()) V3Localize::localizeAll(v3Global.rootp());
+        if (v3Global.opt.fLocalize()) V3Localize::localizeAll(v3Global.rootp());
 
         // Remove remaining scopes; make varrefs/funccalls relative to current module
         V3Descope::descopeAll(v3Global.rootp());
 
         // Icache packing; combine common code in each module's functions into subroutines
-        if (v3Global.opt.oCombine()) V3Combine::combineAll(v3Global.rootp());
+        if (v3Global.opt.fCombine()) V3Combine::combineAll(v3Global.rootp());
     }
 
     V3Error::abortIfErrors();
@@ -452,30 +452,30 @@ static void process() {
     }
 
     // Expand macros and wide operators into C++ primitives
-    if (!v3Global.opt.lintOnly() && !v3Global.opt.xmlOnly() && v3Global.opt.oExpand()) {
+    if (!v3Global.opt.lintOnly() && !v3Global.opt.xmlOnly() && v3Global.opt.fExpand()) {
         V3Expand::expandAll(v3Global.rootp());
     }
 
     // Propagate constants across WORDSEL arrayed temporaries
-    if (!v3Global.opt.xmlOnly() && v3Global.opt.oSubst()) {
+    if (!v3Global.opt.xmlOnly() && v3Global.opt.fSubst()) {
         // Constant folding of expanded stuff
         V3Const::constifyCpp(v3Global.rootp());
         V3Subst::substituteAll(v3Global.rootp());
     }
 
-    if (!v3Global.opt.xmlOnly() && v3Global.opt.oSubstConst()) {
+    if (!v3Global.opt.xmlOnly() && v3Global.opt.fSubstConst()) {
         // Constant folding of substitutions
         V3Const::constifyCpp(v3Global.rootp());
         V3Dead::deadifyAll(v3Global.rootp());
     }
 
     if (!v3Global.opt.lintOnly() && !v3Global.opt.xmlOnly()) {
-        if (v3Global.opt.oMergeCond()) {
+        if (v3Global.opt.fMergeCond()) {
             // Merge conditionals
             V3MergeCond::mergeAll(v3Global.rootp());
         }
 
-        if (v3Global.opt.oReloop()) {
+        if (v3Global.opt.fReloop()) {
             // Reform loops to reduce code size
             // Must be after all Sel/array index based optimizations
             V3Reloop::reloopAll(v3Global.rootp());
diff --git a/test_regress/driver.pl b/test_regress/driver.pl
index fbae94f92..541fb296f 100755
--- a/test_regress/driver.pl
+++ b/test_regress/driver.pl
@@ -77,7 +77,6 @@ my $opt_gdbbt;
 my $opt_gdbsim;
 my $opt_hashset;
 my $opt_jobs = 1;
-my $opt_optimize;
 my $opt_quiet;
 my $opt_rerun;
 my $opt_rrsim;
@@ -104,7 +103,6 @@ if (! GetOptions(
           "hashset=s"   => \$opt_hashset,
           "help"        => \&usage,
           "j=i"         => \$opt_jobs,
-          "optimize:s"  => \$opt_optimize,
           "quiet!"      => \$opt_quiet,
           "rerun!"      => \$opt_rerun,
           "rr!"         => \$opt_rr,
@@ -661,7 +659,7 @@ sub new {
         verilator_define => 'VERILATOR',
         verilator_flags => ["-cc",
                             "-Mdir $self->{obj_dir}",
-                            "-OD",  # As currently disabled unless -O3
+                            "--fdedup",  # As currently disabled unless -O3
                             "--debug-check",
                             "--comp-limit-members 10", ],
         verilator_flags2 => [],
@@ -934,19 +932,6 @@ sub compile_vlt_flags {
         $param{make_main} && $param{verilator_make_gmake};
     unshift @verilator_flags, "../" . $self->{main_filename} if
         $param{make_main} && $param{verilator_make_gmake};
-    if (defined $opt_optimize) {
-        my $letters = "";
-        if ($opt_optimize =~ /[a-zA-Z]/) {
-            $letters = $opt_optimize;
-        } else {  # Randomly turn on/off different optimizations
-            foreach my $l ('a' .. 'z') {
-                $letters .= ((rand() > 0.5) ? $l : uc $l);
-            }
-            unshift @verilator_flags, "--trace" if rand() > 0.5;
-            unshift @verilator_flags, "--coverage" if rand() > 0.5;
-        }
-        unshift @verilator_flags, "--O" . $letters;
-    }
 
     my @cmdargs = (
                    "--prefix " . $param{VM_PREFIX},
@@ -2906,11 +2891,6 @@ Displays this message and program version and exits.
 Run number of parallel tests, or 0 to determine the count based on the
 number of cores installed.  Requires Perl's Parallel::Forker package.
 
-=item --optimize
-
-Randomly turn on/off different optimizations.  With specific flags,
-use those optimization settings
-
 =item --quiet
 
 Suppress all output except for failures and progress messages every 15
diff --git a/test_regress/t/t_altera_lpm_mult_noinl.pl b/test_regress/t/t_altera_lpm_mult_noinl.pl
index 2eac39a3a..63f8aa315 100755
--- a/test_regress/t/t_altera_lpm_mult_noinl.pl
+++ b/test_regress/t/t_altera_lpm_mult_noinl.pl
@@ -15,7 +15,7 @@ top_filename("t/t_altera_lpm.v");
 $module =~ s/_noinl//;
 
 compile(
-    verilator_flags2 => ["--top-module ${module}", "-Oi"]
+    verilator_flags2 => ["--top-module ${module}", "-fno-inline"]
     );
 
 ok(1);
diff --git a/test_regress/t/t_alw_noreorder.pl b/test_regress/t/t_alw_noreorder.pl
index 46d021e6b..edc2a6f7b 100755
--- a/test_regress/t/t_alw_noreorder.pl
+++ b/test_regress/t/t_alw_noreorder.pl
@@ -12,7 +12,7 @@ scenarios(vlt_all => 1);
 
 top_filename("t/t_alw_reorder.v");
 compile(
-    verilator_flags2 => ["--stats -Or"],
+    verilator_flags2 => ["--stats -fno-reorder"],
     );
 
 file_grep($Self->{stats}, qr/Optimizations, Split always\s+(\d+)/i, 0);
diff --git a/test_regress/t/t_assign_inline.pl b/test_regress/t/t_assign_inline.pl
index 27414cae0..1683d1777 100755
--- a/test_regress/t/t_assign_inline.pl
+++ b/test_regress/t/t_assign_inline.pl
@@ -11,7 +11,7 @@ if (!$::Driver) { use FindBin; exec("$FindBin::Bin/bootstrap.pl", @ARGV, $0); di
 scenarios(simulator => 1);
 
 compile(
-    verilator_flags2 => ["-O0 -OG"],
+    verilator_flags2 => ["-O0 -fgate"],
     );
 
 execute(
diff --git a/test_regress/t/t_assign_slice_overflow_ox.pl b/test_regress/t/t_assign_slice_overflow_ox.pl
index 5251be495..8702b94fe 100755
--- a/test_regress/t/t_assign_slice_overflow_ox.pl
+++ b/test_regress/t/t_assign_slice_overflow_ox.pl
@@ -13,7 +13,7 @@ scenarios(simulator => 1);
 top_filename("t_assign_slice_overflow.v");
 
 compile(
-    verilator_flags2 => ["-Ox"],
+    verilator_flags2 => ["-fno-expand"],
     );
 
 execute(
diff --git a/test_regress/t/t_case_66bits_noexpand.pl b/test_regress/t/t_case_66bits_noexpand.pl
index fae2f640f..738da6174 100755
--- a/test_regress/t/t_case_66bits_noexpand.pl
+++ b/test_regress/t/t_case_66bits_noexpand.pl
@@ -13,7 +13,7 @@ scenarios(vlt => 1);
 top_filename("t/t_case_66bits.v");
 
 compile(
-    verilator_flags2 => ['-Ox'],
+    verilator_flags2 => ['-fno-expand'],
     );
 
 execute(
diff --git a/test_regress/t/t_case_incrdecr.pl b/test_regress/t/t_case_incrdecr.pl
index abbcf936a..729c0cc8a 100755
--- a/test_regress/t/t_case_incrdecr.pl
+++ b/test_regress/t/t_case_incrdecr.pl
@@ -11,7 +11,7 @@ if (!$::Driver) { use FindBin; exec("$FindBin::Bin/bootstrap.pl", @ARGV, $0); di
 scenarios(simulator => 1);
 
 compile(
-    verilator_flags2 => ["--trace --Os -x-assign 0"],
+    verilator_flags2 => ["--trace --fno-split -x-assign 0"],
     );
 
 execute(
diff --git a/test_regress/t/t_case_write1_noexpand.pl b/test_regress/t/t_case_write1_noexpand.pl
index cadb667e6..48c57c39a 100755
--- a/test_regress/t/t_case_write1_noexpand.pl
+++ b/test_regress/t/t_case_write1_noexpand.pl
@@ -13,7 +13,7 @@ scenarios(vlt => 1);
 top_filename("t/t_case_write1.v");
 
 compile(
-    verilator_flags2 => ['-Ox'],
+    verilator_flags2 => ['-fno-expand'],
     );
 
 execute(
diff --git a/test_regress/t/t_const_no_opt.pl b/test_regress/t/t_const_no_opt.pl
index 33be39810..79bc15076 100755
--- a/test_regress/t/t_const_no_opt.pl
+++ b/test_regress/t/t_const_no_opt.pl
@@ -13,7 +13,7 @@ top_filename("t/t_const_opt.v");
 
 # Run the same design as t_const_opt.pl without bitopt tree optimization to make sure that the result is same.
 compile(
-    verilator_flags2 => ["-Wno-UNOPTTHREADS", "--stats", "-Oo", "$Self->{t_dir}/t_const_opt.cpp"],
+    verilator_flags2 => ["-Wno-UNOPTTHREADS", "--stats", "-fno-const-bit-op-tree", "$Self->{t_dir}/t_const_opt.cpp"],
     );
 
 execute(
diff --git a/test_regress/t/t_emit_constw.pl b/test_regress/t/t_emit_constw.pl
index 9b1487fcd..8f7895804 100755
--- a/test_regress/t/t_emit_constw.pl
+++ b/test_regress/t/t_emit_constw.pl
@@ -11,7 +11,7 @@ if (!$::Driver) { use FindBin; exec("$FindBin::Bin/bootstrap.pl", @ARGV, $0); di
 scenarios(simulator => 1);
 
 compile(
-    verilator_flags2 => ['--Ox'],
+    verilator_flags2 => ['--fno-expand'],
     );
 
 execute(
diff --git a/test_regress/t/t_func_twocall_noexpand.pl b/test_regress/t/t_func_twocall_noexpand.pl
index 001824bc6..452d4b37a 100755
--- a/test_regress/t/t_func_twocall_noexpand.pl
+++ b/test_regress/t/t_func_twocall_noexpand.pl
@@ -13,7 +13,7 @@ scenarios(vlt => 1);
 top_filename("t/t_func_twocall.v");
 
 compile(
-    verilator_flags2 => ['-Ox'],
+    verilator_flags2 => ['-fno-expand'],
     );
 
 execute(
diff --git a/test_regress/t/t_gen_genblk_noinl.pl b/test_regress/t/t_gen_genblk_noinl.pl
index 7574a1cfb..ef537cd4d 100755
--- a/test_regress/t/t_gen_genblk_noinl.pl
+++ b/test_regress/t/t_gen_genblk_noinl.pl
@@ -16,7 +16,7 @@ scenarios(simulator => 1);
 $Self->{sim_time} = 11000;
 
 compile(
-    v_flags2 => ["-Oi"],
+    v_flags2 => ["-fno-inline"],
     );
 
 execute(
diff --git a/test_regress/t/t_incr_void.pl b/test_regress/t/t_incr_void.pl
index 5b95e5b74..e7d3e18e3 100755
--- a/test_regress/t/t_incr_void.pl
+++ b/test_regress/t/t_incr_void.pl
@@ -11,7 +11,7 @@ if (!$::Driver) { use FindBin; exec("$FindBin::Bin/bootstrap.pl", @ARGV, $0); di
 scenarios(simulator => 1);
 
 compile(
-    verilator_flags2 => ["--Os -x-assign 0"],
+    verilator_flags2 => ["--fno-split -x-assign 0"],
     );
 
 execute(
diff --git a/test_regress/t/t_inst_slice_noinl.pl b/test_regress/t/t_inst_slice_noinl.pl
index 11f75c752..aa56e6155 100755
--- a/test_regress/t/t_inst_slice_noinl.pl
+++ b/test_regress/t/t_inst_slice_noinl.pl
@@ -13,7 +13,7 @@ scenarios(simulator => 1);
 top_filename("t/t_inst_slice.v");
 
 compile(
-    v_flags2 => ["-Oi"],
+    v_flags2 => ["-fno-inline"],
     );
 
 execute(
diff --git a/test_regress/t/t_interface1_modport_noinl.pl b/test_regress/t/t_interface1_modport_noinl.pl
index b077bef4e..4f4b314ae 100755
--- a/test_regress/t/t_interface1_modport_noinl.pl
+++ b/test_regress/t/t_interface1_modport_noinl.pl
@@ -13,7 +13,7 @@ scenarios(simulator => 1);
 top_filename("t/t_interface1_modport.v");
 
 compile(
-    v_flags2 => ["-Oi"],
+    v_flags2 => ["-fno-inline"],
     );
 
 execute(
diff --git a/test_regress/t/t_interface1_noinl.pl b/test_regress/t/t_interface1_noinl.pl
index 3c9d8d316..867b1e993 100755
--- a/test_regress/t/t_interface1_noinl.pl
+++ b/test_regress/t/t_interface1_noinl.pl
@@ -13,7 +13,7 @@ scenarios(simulator => 1);
 top_filename("t/t_interface1.v");
 
 compile(
-    v_flags2 => ["-Oi"],
+    v_flags2 => ["-fno-inline"],
     );
 
 execute(
diff --git a/test_regress/t/t_interface2_noinl.pl b/test_regress/t/t_interface2_noinl.pl
index 57b72e7a7..cad1b6e3d 100755
--- a/test_regress/t/t_interface2_noinl.pl
+++ b/test_regress/t/t_interface2_noinl.pl
@@ -13,7 +13,7 @@ scenarios(simulator => 1);
 top_filename("t/t_interface2.v");
 
 compile(
-    verilator_flags2 => ["--top-module t -Oi"],
+    verilator_flags2 => ["--top-module t -fno-inline"],
     );
 
 execute(
diff --git a/test_regress/t/t_interface_array2_noinl.pl b/test_regress/t/t_interface_array2_noinl.pl
index ad389d0fb..7bf1518f5 100755
--- a/test_regress/t/t_interface_array2_noinl.pl
+++ b/test_regress/t/t_interface_array2_noinl.pl
@@ -13,7 +13,7 @@ scenarios(simulator => 1);
 top_filename("t/t_interface_array2.v");
 
 compile(
-    v_flags2 => ["-Oi"],
+    v_flags2 => ["-fno-inline"],
     );
 
 execute(
diff --git a/test_regress/t/t_interface_array_noinl.pl b/test_regress/t/t_interface_array_noinl.pl
index 02bf8fd89..df71f77e9 100755
--- a/test_regress/t/t_interface_array_noinl.pl
+++ b/test_regress/t/t_interface_array_noinl.pl
@@ -13,7 +13,7 @@ scenarios(simulator => 1);
 top_filename("t/t_interface_array.v");
 
 compile(
-    v_flags2 => ["-Oi"],
+    v_flags2 => ["-fno-inline"],
     );
 
 execute(
diff --git a/test_regress/t/t_interface_down_noinl.pl b/test_regress/t/t_interface_down_noinl.pl
index fb03fc988..34ce5cb69 100755
--- a/test_regress/t/t_interface_down_noinl.pl
+++ b/test_regress/t/t_interface_down_noinl.pl
@@ -13,7 +13,7 @@ scenarios(simulator => 1);
 top_filename("t/t_interface_down.v");
 
 compile(
-    v_flags2 => ["-Oi"],
+    v_flags2 => ["-fno-inline"],
     );
 
 execute(
diff --git a/test_regress/t/t_interface_gen10_noinl.pl b/test_regress/t/t_interface_gen10_noinl.pl
index e5c3f22c5..f691c6d0a 100755
--- a/test_regress/t/t_interface_gen10_noinl.pl
+++ b/test_regress/t/t_interface_gen10_noinl.pl
@@ -13,7 +13,7 @@ scenarios(simulator => 1);
 top_filename("t/t_interface_gen10.v");
 
 compile(
-    v_flags2 => ["-Oi"],
+    v_flags2 => ["-fno-inline"],
     );
 
 execute(
diff --git a/test_regress/t/t_interface_gen11_noinl.pl b/test_regress/t/t_interface_gen11_noinl.pl
index 82a6a9a27..d1e7dd3c0 100755
--- a/test_regress/t/t_interface_gen11_noinl.pl
+++ b/test_regress/t/t_interface_gen11_noinl.pl
@@ -13,7 +13,7 @@ scenarios(simulator => 1);
 top_filename("t/t_interface_gen11.v");
 
 compile(
-    v_flags2 => ["-Oi"],
+    v_flags2 => ["-fno-inline"],
     );
 
 execute(
diff --git a/test_regress/t/t_interface_gen12_noinl.pl b/test_regress/t/t_interface_gen12_noinl.pl
index c3f59ba19..8ebecd448 100755
--- a/test_regress/t/t_interface_gen12_noinl.pl
+++ b/test_regress/t/t_interface_gen12_noinl.pl
@@ -13,7 +13,7 @@ scenarios(simulator => 1);
 top_filename("t/t_interface_gen12.v");
 
 compile(
-    v_flags2 => ["-Oi"],
+    v_flags2 => ["-fno-inline"],
     );
 
 execute(
diff --git a/test_regress/t/t_interface_gen2_noinl.pl b/test_regress/t/t_interface_gen2_noinl.pl
index fc7c4bfb1..eb772bab6 100755
--- a/test_regress/t/t_interface_gen2_noinl.pl
+++ b/test_regress/t/t_interface_gen2_noinl.pl
@@ -13,7 +13,7 @@ scenarios(simulator => 1);
 top_filename("t/t_interface_gen2.v");
 
 compile(
-    v_flags2 => ["-Oi"],
+    v_flags2 => ["-fno-inline"],
     );
 
 execute(
diff --git a/test_regress/t/t_interface_gen3_noinl.pl b/test_regress/t/t_interface_gen3_noinl.pl
index e49dfc39a..b63c72eb9 100755
--- a/test_regress/t/t_interface_gen3_noinl.pl
+++ b/test_regress/t/t_interface_gen3_noinl.pl
@@ -13,7 +13,7 @@ scenarios(simulator => 1);
 top_filename("t/t_interface_gen3.v");
 
 compile(
-    v_flags2 => ["-Oi"],
+    v_flags2 => ["-fno-inline"],
     );
 
 execute(
diff --git a/test_regress/t/t_interface_gen4_noinl.pl b/test_regress/t/t_interface_gen4_noinl.pl
index 4a0b00930..e724c2859 100755
--- a/test_regress/t/t_interface_gen4_noinl.pl
+++ b/test_regress/t/t_interface_gen4_noinl.pl
@@ -13,7 +13,7 @@ scenarios(simulator => 1);
 top_filename("t/t_interface_gen4.v");
 
 compile(
-    v_flags2 => ["-Oi"],
+    v_flags2 => ["-fno-inline"],
     );
 
 execute(
diff --git a/test_regress/t/t_interface_gen5_noinl.pl b/test_regress/t/t_interface_gen5_noinl.pl
index 0873ce9c5..5b4852691 100755
--- a/test_regress/t/t_interface_gen5_noinl.pl
+++ b/test_regress/t/t_interface_gen5_noinl.pl
@@ -13,7 +13,7 @@ scenarios(simulator => 1);
 top_filename("t/t_interface_gen5.v");
 
 compile(
-    v_flags2 => ["-Oi"],
+    v_flags2 => ["-fno-inline"],
     );
 
 execute(
diff --git a/test_regress/t/t_interface_gen6_noinl.pl b/test_regress/t/t_interface_gen6_noinl.pl
index 4c42c6797..e43d9460a 100755
--- a/test_regress/t/t_interface_gen6_noinl.pl
+++ b/test_regress/t/t_interface_gen6_noinl.pl
@@ -13,7 +13,7 @@ scenarios(simulator => 1);
 top_filename("t/t_interface_gen6.v");
 
 compile(
-    v_flags2 => ["-Oi"],
+    v_flags2 => ["-fno-inline"],
     );
 
 execute(
diff --git a/test_regress/t/t_interface_gen7_noinl.pl b/test_regress/t/t_interface_gen7_noinl.pl
index 27cb3ea61..458c5f0f6 100755
--- a/test_regress/t/t_interface_gen7_noinl.pl
+++ b/test_regress/t/t_interface_gen7_noinl.pl
@@ -13,7 +13,7 @@ scenarios(simulator => 1);
 top_filename("t/t_interface_gen7.v");
 
 compile(
-    v_flags2 => ["-Oi"],
+    v_flags2 => ["-fno-inline"],
     );
 
 execute(
diff --git a/test_regress/t/t_interface_gen8_noinl.pl b/test_regress/t/t_interface_gen8_noinl.pl
index ba3b2b132..644d9a10e 100755
--- a/test_regress/t/t_interface_gen8_noinl.pl
+++ b/test_regress/t/t_interface_gen8_noinl.pl
@@ -13,7 +13,7 @@ scenarios(simulator => 1);
 top_filename("t/t_interface_gen8.v");
 
 compile(
-    v_flags2 => ["-Oi"],
+    v_flags2 => ["-fno-inline"],
     );
 
 execute(
diff --git a/test_regress/t/t_interface_gen9_noinl.pl b/test_regress/t/t_interface_gen9_noinl.pl
index 48f4eb8be..6ac0d6296 100755
--- a/test_regress/t/t_interface_gen9_noinl.pl
+++ b/test_regress/t/t_interface_gen9_noinl.pl
@@ -13,7 +13,7 @@ scenarios(simulator => 1);
 top_filename("t/t_interface_gen9.v");
 
 compile(
-    v_flags2 => ["-Oi"],
+    v_flags2 => ["-fno-inline"],
     );
 
 execute(
diff --git a/test_regress/t/t_interface_gen_noinl.pl b/test_regress/t/t_interface_gen_noinl.pl
index 5813d42eb..17273106f 100755
--- a/test_regress/t/t_interface_gen_noinl.pl
+++ b/test_regress/t/t_interface_gen_noinl.pl
@@ -13,7 +13,7 @@ scenarios(simulator => 1);
 top_filename("t/t_interface_gen.v");
 
 compile(
-    v_flags2 => ["-Oi"],
+    v_flags2 => ["-fno-inline"],
     );
 
 execute(
diff --git a/test_regress/t/t_interface_inl.pl b/test_regress/t/t_interface_inl.pl
index efb67ed7f..08dfa385c 100755
--- a/test_regress/t/t_interface_inl.pl
+++ b/test_regress/t/t_interface_inl.pl
@@ -14,7 +14,7 @@ top_filename("t/t_interface.v");
 
 compile(
     # Avoid inlining so we find bugs in the non-inliner connection code
-    verilator_flags2 => ["-Oi"],
+    verilator_flags2 => ["-fno-inline"],
     );
 
 execute(
diff --git a/test_regress/t/t_interface_modport_import_noinl.pl b/test_regress/t/t_interface_modport_import_noinl.pl
index 3821fef11..a9e97bee1 100755
--- a/test_regress/t/t_interface_modport_import_noinl.pl
+++ b/test_regress/t/t_interface_modport_import_noinl.pl
@@ -13,7 +13,7 @@ scenarios(simulator => 1);
 top_filename("t/t_interface_modport_import.v");
 
 compile(
-    v_flags2 => ["-Oi"],
+    v_flags2 => ["-fno-inline"],
     );
 
 execute(
diff --git a/test_regress/t/t_interface_modport_inl.pl b/test_regress/t/t_interface_modport_inl.pl
index 9afcd9cdd..eb2ca2181 100755
--- a/test_regress/t/t_interface_modport_inl.pl
+++ b/test_regress/t/t_interface_modport_inl.pl
@@ -14,7 +14,7 @@ top_filename("t/t_interface_modport.v");
 
 compile(
     # Avoid inlining so we find bugs in the non-inliner connection code
-    verilator_flags2 => ["-Oi"],
+    verilator_flags2 => ["-fno-inline"],
     );
 
 execute(
diff --git a/test_regress/t/t_interface_modport_noinl.pl b/test_regress/t/t_interface_modport_noinl.pl
index 4c051df1a..7f1015d23 100755
--- a/test_regress/t/t_interface_modport_noinl.pl
+++ b/test_regress/t/t_interface_modport_noinl.pl
@@ -13,7 +13,7 @@ scenarios(simulator => 1);
 top_filename("t/t_interface_modport.v");
 
 compile(
-    v_flags2 => ["-Oi"],
+    v_flags2 => ["-fno-inline"],
     );
 
 execute(
diff --git a/test_regress/t/t_interface_mp_func_noinl.pl b/test_regress/t/t_interface_mp_func_noinl.pl
index 432a7308a..89f4835b5 100755
--- a/test_regress/t/t_interface_mp_func_noinl.pl
+++ b/test_regress/t/t_interface_mp_func_noinl.pl
@@ -13,7 +13,7 @@ scenarios(simulator => 1);
 top_filename("t/t_interface_mp_func.v");
 
 compile(
-    v_flags2 => ["-Oi"],
+    v_flags2 => ["-fno-inline"],
     );
 
 execute(
diff --git a/test_regress/t/t_interface_nest_noinl.pl b/test_regress/t/t_interface_nest_noinl.pl
index 9d88a39a0..e042d33c1 100755
--- a/test_regress/t/t_interface_nest_noinl.pl
+++ b/test_regress/t/t_interface_nest_noinl.pl
@@ -13,7 +13,7 @@ scenarios(simulator => 1);
 top_filename("t/t_interface_nest.v");
 
 compile(
-    v_flags2 => ["-Oi"],
+    v_flags2 => ["-fno-inline"],
     );
 
 execute(
diff --git a/test_regress/t/t_interface_noinl.pl b/test_regress/t/t_interface_noinl.pl
index 52cb09c98..7be6235ad 100755
--- a/test_regress/t/t_interface_noinl.pl
+++ b/test_regress/t/t_interface_noinl.pl
@@ -13,7 +13,7 @@ scenarios(simulator => 1);
 top_filename("t/t_interface.v");
 
 compile(
-    v_flags2 => ["-Oi"],
+    v_flags2 => ["-fno-inline"],
     );
 
 execute(
diff --git a/test_regress/t/t_interface_twod_noinl.pl b/test_regress/t/t_interface_twod_noinl.pl
index 18f0adf62..e77089cb0 100755
--- a/test_regress/t/t_interface_twod_noinl.pl
+++ b/test_regress/t/t_interface_twod_noinl.pl
@@ -13,7 +13,7 @@ scenarios(simulator => 1);
 top_filename("t/t_interface_twod.v");
 
 compile(
-    v_flags2 => ["-Oi"],
+    v_flags2 => ["-fno-inline"],
     );
 
 execute(
diff --git a/test_regress/t/t_lint_setout_bad_noinl.pl b/test_regress/t/t_lint_setout_bad_noinl.pl
index 4b5131821..cbbf96bb4 100755
--- a/test_regress/t/t_lint_setout_bad_noinl.pl
+++ b/test_regress/t/t_lint_setout_bad_noinl.pl
@@ -13,7 +13,7 @@ scenarios(linter => 1);
 top_filename("t/t_lint_setout_bad.v");
 
 lint(
-    verilator_flags2 => ["--lint-only -Oi"],
+    verilator_flags2 => ["--lint-only -fno-inline"],
     fails => 1,
     expect_filename => $Self->{golden_filename},
     );
diff --git a/test_regress/t/t_math_cond_huge_noexpand.pl b/test_regress/t/t_math_cond_huge_noexpand.pl
index 0ae4e3ce4..15399cb9f 100755
--- a/test_regress/t/t_math_cond_huge_noexpand.pl
+++ b/test_regress/t/t_math_cond_huge_noexpand.pl
@@ -13,7 +13,7 @@ scenarios(vlt => 1);
 top_filename("t/t_math_cond_huge.v");
 
 compile(
-    verilator_flags2 => ['-Ox'],
+    verilator_flags2 => ['-fno-expand'],
     );
 
 execute(
diff --git a/test_regress/t/t_math_div_noexpand.pl b/test_regress/t/t_math_div_noexpand.pl
index 4dbcba15c..fa7ecd2ec 100755
--- a/test_regress/t/t_math_div_noexpand.pl
+++ b/test_regress/t/t_math_div_noexpand.pl
@@ -13,7 +13,7 @@ scenarios(vlt => 1);
 top_filename("t/t_math_div.v");
 
 compile(
-    verilator_flags2 => ['-Ox'],
+    verilator_flags2 => ['-fno-expand'],
     );
 
 execute(
diff --git a/test_regress/t/t_math_eq_noexpand.pl b/test_regress/t/t_math_eq_noexpand.pl
index f8b2375c0..2c3907b70 100755
--- a/test_regress/t/t_math_eq_noexpand.pl
+++ b/test_regress/t/t_math_eq_noexpand.pl
@@ -13,7 +13,7 @@ scenarios(vlt => 1);
 top_filename("t/t_math_eq.v");
 
 compile(
-    verilator_flags2 => ['-Ox'],
+    verilator_flags2 => ['-fno-expand'],
     );
 
 execute(
diff --git a/test_regress/t/t_math_red_noexpand.pl b/test_regress/t/t_math_red_noexpand.pl
index 89e54c0c9..655ce0246 100755
--- a/test_regress/t/t_math_red_noexpand.pl
+++ b/test_regress/t/t_math_red_noexpand.pl
@@ -13,7 +13,7 @@ scenarios(vlt => 1);
 top_filename("t/t_math_red.v");
 
 compile(
-    verilator_flags2 => ['-Ox'],
+    verilator_flags2 => ['-fno-expand'],
     );
 
 execute(
diff --git a/test_regress/t/t_math_shift_noexpand.pl b/test_regress/t/t_math_shift_noexpand.pl
index e8a59556b..8584197a0 100755
--- a/test_regress/t/t_math_shift_noexpand.pl
+++ b/test_regress/t/t_math_shift_noexpand.pl
@@ -13,7 +13,7 @@ scenarios(vlt => 1);
 top_filename("t/t_math_shift.v");
 
 compile(
-    verilator_flags2 => ["-Wno-CLKDATA", '-Ox'],
+    verilator_flags2 => ["-Wno-CLKDATA", '-fno-expand'],
     );
 
 execute(
diff --git a/test_regress/t/t_math_signed_noexpand.pl b/test_regress/t/t_math_signed_noexpand.pl
index 336d35594..b086af557 100755
--- a/test_regress/t/t_math_signed_noexpand.pl
+++ b/test_regress/t/t_math_signed_noexpand.pl
@@ -13,7 +13,7 @@ scenarios(vlt => 1);
 top_filename("t/t_math_signed.v");
 
 compile(
-    verilator_flags2 => ['-Ox'],
+    verilator_flags2 => ['-fno-expand'],
     );
 
 execute(
diff --git a/test_regress/t/t_math_vliw_noexpand.pl b/test_regress/t/t_math_vliw_noexpand.pl
index fce202e04..5ca1e425f 100755
--- a/test_regress/t/t_math_vliw_noexpand.pl
+++ b/test_regress/t/t_math_vliw_noexpand.pl
@@ -13,7 +13,7 @@ scenarios(vlt => 1);
 top_filename("t/t_math_vliw.v");
 
 compile(
-    verilator_flags2 => ['-Ox'],
+    verilator_flags2 => ['-fno-expand'],
     );
 
 execute(
diff --git a/test_regress/t/t_mem_multi_io.pl b/test_regress/t/t_mem_multi_io.pl
index 1691d75f1..4e371f1d7 100755
--- a/test_regress/t/t_mem_multi_io.pl
+++ b/test_regress/t/t_mem_multi_io.pl
@@ -12,7 +12,7 @@ scenarios(simulator => 1);
 
 compile(
     # Disable inlining, this test is trivial without it
-    verilator_flags2 => ["-Oi --trace"],
+    verilator_flags2 => ["-fno-inline --trace"],
     verilator_flags3 => [],
     );
 
diff --git a/test_regress/t/t_mem_multi_io2_cc.pl b/test_regress/t/t_mem_multi_io2_cc.pl
index 3edda698b..bfd551aed 100755
--- a/test_regress/t/t_mem_multi_io2_cc.pl
+++ b/test_regress/t/t_mem_multi_io2_cc.pl
@@ -15,7 +15,7 @@ top_filename("t/t_mem_multi_io2.v");
 compile(
     make_top_shell => 0,
     make_main => 0,
-    verilator_flags2 => ["--exe $Self->{t_dir}/t_mem_multi_io2.cpp -Oi"],
+    verilator_flags2 => ["--exe $Self->{t_dir}/t_mem_multi_io2.cpp -fno-inline"],
     verilator_flags3 => [],
     );
 
diff --git a/test_regress/t/t_mem_multi_io2_sc.pl b/test_regress/t/t_mem_multi_io2_sc.pl
index 11ae8cbfc..2fb4bf70c 100755
--- a/test_regress/t/t_mem_multi_io2_sc.pl
+++ b/test_regress/t/t_mem_multi_io2_sc.pl
@@ -15,7 +15,7 @@ top_filename("t/t_mem_multi_io2.v");
 compile(
     make_top_shell => 0,
     make_main => 0,
-    verilator_flags2 => ["--exe $Self->{t_dir}/t_mem_multi_io2.cpp --sc -Oi"],
+    verilator_flags2 => ["--exe $Self->{t_dir}/t_mem_multi_io2.cpp --sc -fno-inline"],
     );
 
 execute(
diff --git a/test_regress/t/t_mem_multi_io3_cc.pl b/test_regress/t/t_mem_multi_io3_cc.pl
index 4ad019dbf..b6090a775 100755
--- a/test_regress/t/t_mem_multi_io3_cc.pl
+++ b/test_regress/t/t_mem_multi_io3_cc.pl
@@ -15,7 +15,7 @@ top_filename("t/t_mem_multi_io3.v");
 compile(
     make_top_shell => 0,
     make_main => 0,
-    verilator_flags2 => ["--exe $Self->{t_dir}/t_mem_multi_io3.cpp -Oi"],
+    verilator_flags2 => ["--exe $Self->{t_dir}/t_mem_multi_io3.cpp -fno-inline"],
     verilator_flags3 => [],
     );
 
diff --git a/test_regress/t/t_mem_multi_io3_sc.pl b/test_regress/t/t_mem_multi_io3_sc.pl
index 5825c7845..f37d9dedd 100755
--- a/test_regress/t/t_mem_multi_io3_sc.pl
+++ b/test_regress/t/t_mem_multi_io3_sc.pl
@@ -15,7 +15,7 @@ top_filename("t/t_mem_multi_io3.v");
 compile(
     make_top_shell => 0,
     make_main => 0,
-    verilator_flags2 => ["--exe $Self->{t_dir}/t_mem_multi_io3.cpp --sc -Oi"],
+    verilator_flags2 => ["--exe $Self->{t_dir}/t_mem_multi_io3.cpp --sc -fno-inline"],
     verilator_flags3 => [],
     );
 
diff --git a/test_regress/t/t_mem_multidim_Ox.pl b/test_regress/t/t_mem_multidim_Ox.pl
index bb4dbc122..ccde0bbbd 100755
--- a/test_regress/t/t_mem_multidim_Ox.pl
+++ b/test_regress/t/t_mem_multidim_Ox.pl
@@ -13,7 +13,7 @@ scenarios(simulator => 1);
 top_filename("t/t_mem_multidim.v");
 
 compile(
-    verilator_flags2 => ['--Ox'],
+    verilator_flags2 => ['--fno-expand'],
     );
 
 execute(
diff --git a/test_regress/t/t_mem_packed_noexpand.pl b/test_regress/t/t_mem_packed_noexpand.pl
index d5fc2b5da..df4c82d6d 100755
--- a/test_regress/t/t_mem_packed_noexpand.pl
+++ b/test_regress/t/t_mem_packed_noexpand.pl
@@ -13,7 +13,7 @@ scenarios(vlt => 1);
 top_filename("t/t_mem_packed.v");
 
 compile(
-    verilator_flags2 => ['-Ox'],
+    verilator_flags2 => ['-fno-expand'],
     );
 
 execute(
diff --git a/test_regress/t/t_mod_interface_array0_noinl.pl b/test_regress/t/t_mod_interface_array0_noinl.pl
index 3c74fd016..56032e0d9 100755
--- a/test_regress/t/t_mod_interface_array0_noinl.pl
+++ b/test_regress/t/t_mod_interface_array0_noinl.pl
@@ -13,7 +13,7 @@ scenarios(simulator => 1);
 top_filename("t/t_mod_interface_array0.v");
 
 compile(
-    v_flags2 => ["-Oi"],
+    v_flags2 => ["-fno-inline"],
     );
 
 execute(
diff --git a/test_regress/t/t_mod_interface_array1_noinl.pl b/test_regress/t/t_mod_interface_array1_noinl.pl
index 34871282a..651bb1c65 100755
--- a/test_regress/t/t_mod_interface_array1_noinl.pl
+++ b/test_regress/t/t_mod_interface_array1_noinl.pl
@@ -13,7 +13,7 @@ scenarios(simulator => 1);
 top_filename("t/t_mod_interface_array1.v");
 
 compile(
-    v_flags2 => ["-Oi"],
+    v_flags2 => ["-fno-inline"],
     );
 
 execute(
diff --git a/test_regress/t/t_mod_interface_array2_noinl.pl b/test_regress/t/t_mod_interface_array2_noinl.pl
index c19612e57..2afa9e020 100755
--- a/test_regress/t/t_mod_interface_array2_noinl.pl
+++ b/test_regress/t/t_mod_interface_array2_noinl.pl
@@ -13,7 +13,7 @@ scenarios(simulator => 1);
 top_filename("t/t_mod_interface_array2.v");
 
 compile(
-    v_flags2 => ["-Oi"],
+    v_flags2 => ["-fno-inline"],
     );
 
 execute(
diff --git a/test_regress/t/t_mod_interface_array4_noinl.pl b/test_regress/t/t_mod_interface_array4_noinl.pl
index 6797c1016..62ad2ca24 100755
--- a/test_regress/t/t_mod_interface_array4_noinl.pl
+++ b/test_regress/t/t_mod_interface_array4_noinl.pl
@@ -13,7 +13,7 @@ scenarios(simulator => 1);
 top_filename("t/t_mod_interface_array4.v");
 
 compile(
-    v_flags2 => ["-Oi"],
+    v_flags2 => ["-fno-inline"],
     );
 
 execute(
diff --git a/test_regress/t/t_mod_interface_array6_noinl.pl b/test_regress/t/t_mod_interface_array6_noinl.pl
index 5244ac42c..f07ea1917 100755
--- a/test_regress/t/t_mod_interface_array6_noinl.pl
+++ b/test_regress/t/t_mod_interface_array6_noinl.pl
@@ -13,7 +13,7 @@ scenarios(simulator => 1);
 top_filename("t/t_mod_interface_array6.v");
 
 compile(
-    v_flags2 => ["-Oi"],
+    v_flags2 => ["-fno-inline"],
     );
 
 execute(
diff --git a/test_regress/t/t_optm_if_cond.pl b/test_regress/t/t_optm_if_cond.pl
index b67f09305..7910f570f 100755
--- a/test_regress/t/t_optm_if_cond.pl
+++ b/test_regress/t/t_optm_if_cond.pl
@@ -11,7 +11,7 @@ if (!$::Driver) { use FindBin; exec("$FindBin::Bin/bootstrap.pl", @ARGV, $0); di
 scenarios(vlt => 1);
 
 compile(
-    verilator_flags2 => ['--stats', "-Ow"],
+    verilator_flags2 => ['--stats', "-fno-merge-cond"],
     );
 
 if ($Self->{vlt_all}) {
diff --git a/test_regress/t/t_var_assign_landr_noexpand.pl b/test_regress/t/t_var_assign_landr_noexpand.pl
index cd058334d..e616f77c3 100755
--- a/test_regress/t/t_var_assign_landr_noexpand.pl
+++ b/test_regress/t/t_var_assign_landr_noexpand.pl
@@ -13,7 +13,7 @@ scenarios(vlt => 1);
 top_filename("t/t_var_assign_landr.v");
 
 compile(
-    verilator_flags2 => ['-Ox'],
+    verilator_flags2 => ['-fno-expand'],
     );
 
 execute(