diff --git a/flang/include/flang/Optimizer/Transforms/Passes.td b/flang/include/flang/Optimizer/Transforms/Passes.td
--- a/flang/include/flang/Optimizer/Transforms/Passes.td
+++ b/flang/include/flang/Optimizer/Transforms/Passes.td
@@ -172,7 +172,7 @@
   }];
   let constructor = "::fir::createExternalNameConversionPass()";
   let options = [
-    Option<"appendUnderscore", "append-underscore",
+    Option<"appendUnderscoreOpt", "append-underscore",
            "bool", /*default=*/"true",
            "Append trailing underscore to external names.">
   ];
diff --git a/flang/lib/Optimizer/Transforms/ExternalNameConversion.cpp b/flang/lib/Optimizer/Transforms/ExternalNameConversion.cpp
--- a/flang/lib/Optimizer/Transforms/ExternalNameConversion.cpp
+++ b/flang/lib/Optimizer/Transforms/ExternalNameConversion.cpp
@@ -158,13 +158,14 @@
   ExternalNameConversionPass(bool appendUnderscoring)
       : appendUnderscores(appendUnderscoring) {}
 
-  ExternalNameConversionPass() { appendUnderscores = appendUnderscore; }
+  ExternalNameConversionPass() { usePassOpt = true; }
 
   mlir::ModuleOp getModule() { return getOperation(); }
   void runOnOperation() override;
 
 private:
   bool appendUnderscores;
+  bool usePassOpt;
 };
 } // namespace
 
@@ -172,9 +173,11 @@
   auto op = getOperation();
   auto *context = &getContext();
 
+  appendUnderscores = (usePassOpt) ? appendUnderscoreOpt : appendUnderscores;
+
   mlir::RewritePatternSet patterns(context);
   patterns.insert<MangleNameOnFuncOp, MangleNameForCommonBlock,
-                  MangleNameOnAddrOfOp>(context, appendUnderscore);
+                  MangleNameOnAddrOfOp>(context, appendUnderscores);
 
   ConversionTarget target(*context);
   target.addLegalDialect<fir::FIROpsDialect, LLVM::LLVMDialect,
diff --git a/flang/test/Driver/underscoring.f90 b/flang/test/Driver/underscoring.f90
--- a/flang/test/Driver/underscoring.f90
+++ b/flang/test/Driver/underscoring.f90
@@ -10,15 +10,15 @@
 end
 
 ! UNDERSCORING: test_
+! UNDERSCORING-NOT: {{test:$}}
 ! UNDERSCORING: ext_sub_
+! UNDERSCORING-NOT: {{ext_sub[^_]*$}}
 ! UNDERSCORING: comblk_
+! UNDERSCORING-NOT: comblk,
 
 ! NO-UNDERSCORING-NOT: test_
-! NO-UNDERSCORING-NOT: _QPtest
-! NO-UNDERSCORING: test
+! NO-UNDERSCORING: test:
 ! NO-UNDERSCORING-NOT: ext_sub_
-! NO-UNDERSCORING-NOT: _QPext_sub
-! NO-UNDERSCORING: ext_sub
+! NO-UNDERSCORING: {{ext_sub[^_]*$}}
 ! NO-UNDERSCORING-NOT: comblk_
-! NO-UNDERSCORING-NOT: _QBcomblk
-! NO-UNDERSCORING: comblk
+! NO-UNDERSCORING: comblk,