diff --git a/tools/ccc/ccclib/Tools.py b/tools/ccc/ccclib/Tools.py
index 13d4a54d85..4b600af448 100644
--- a/tools/ccc/ccclib/Tools.py
+++ b/tools/ccc/ccclib/Tools.py
@@ -307,7 +307,14 @@ class Clang_CompileTool(Tool):
 
         arglist.addAllArgs(cmd_args, arglist.parser.fblocksGroup)
 
-        arglist.addAllArgs(cmd_args, arglist.parser.OOption)
+        # Manually translate -O to -O1; let clang reject others.
+        arg = arglist.getLastArg(arglist.parser.OOption)
+        if arg:
+            if arglist.getValue(arg) == '':
+                cmd_args.append('-O1')
+            else:
+                cmd_args.extend(arglist.render(arg))
+
         arglist.addAllArgs2(cmd_args, arglist.parser.ClangWGroup, arglist.parser.pedanticGroup)
         arglist.addLastArg(cmd_args, arglist.parser.wOption)
         arglist.addAllArgs3(cmd_args, arglist.parser.stdOption, arglist.parser.ansiOption, arglist.parser.trigraphsOption)
diff --git a/tools/ccc/test/ccc/O.c b/tools/ccc/test/ccc/O.c
new file mode 100644
index 0000000000..7ef0aba61d
--- /dev/null
+++ b/tools/ccc/test/ccc/O.c
@@ -0,0 +1,4 @@
+// Just check that clang accepts these.
+
+// RUN: xcc -fsyntax-only -O1 -O2 %s &&
+// RUN: xcc -fsyntax-only -O %s