diff --git a/clang/include/clang/Driver/Options.td b/clang/include/clang/Driver/Options.td
--- a/clang/include/clang/Driver/Options.td
+++ b/clang/include/clang/Driver/Options.td
@@ -400,7 +400,7 @@
 def ObjC : Flag<["-"], "ObjC">, Flags<[DriverOption]>,
   HelpText<"Treat source input files as Objective-C inputs">;
 def O : Joined<["-"], "O">, Group<O_Group>, Flags<[CC1Option]>;
-def O_flag : Flag<["-"], "O">, Flags<[CC1Option]>, Alias<O>, AliasArgs<["2"]>;
+def O_flag : Flag<["-"], "O">, Flags<[CC1Option]>, Alias<O>, AliasArgs<["1"]>;
 def Ofast : Joined<["-"], "Ofast">, Group<O_Group>, Flags<[CC1Option]>;
 def P : Flag<["-"], "P">, Flags<[CC1Option]>, Group<Preprocessor_Group>,
   HelpText<"Disable linemarker output in -E mode">;
diff --git a/clang/lib/Frontend/CompilerInvocation.cpp b/clang/lib/Frontend/CompilerInvocation.cpp
--- a/clang/lib/Frontend/CompilerInvocation.cpp
+++ b/clang/lib/Frontend/CompilerInvocation.cpp
@@ -137,7 +137,7 @@
     assert(A->getOption().matches(options::OPT_O));
 
     StringRef S(A->getValue());
-    if (S == "s" || S == "z" || S.empty())
+    if (S == "s" || S == "z")
       return llvm::CodeGenOpt::Default;
 
     if (S == "g")
diff --git a/clang/test/Driver/O.c b/clang/test/Driver/O.c
--- a/clang/test/Driver/O.c
+++ b/clang/test/Driver/O.c
@@ -1,7 +1,7 @@
 // Test that we parse and translate the -O option correctly.
 
 // RUN: %clang -O -### %s 2>&1 | FileCheck -check-prefix=CHECK-O %s
-// CHECK-O: -O2
+// CHECK-O: -O1
 
 // RUN: %clang -O0 -### %s 2>&1 | FileCheck -check-prefix=CHECK-O0 %s
 // CHECK-O0: -O0
diff --git a/clang/test/Driver/clang_f_opts.c b/clang/test/Driver/clang_f_opts.c
--- a/clang/test/Driver/clang_f_opts.c
+++ b/clang/test/Driver/clang_f_opts.c
@@ -135,7 +135,7 @@
 // RUN: %clang -### -S -fno-tree-vectorize -fvectorize %s 2>&1 | FileCheck -check-prefix=CHECK-VECTORIZE %s
 // RUN: %clang -### -S -fno-tree-vectorize %s 2>&1 | FileCheck -check-prefix=CHECK-NO-VECTORIZE %s
 // RUN: %clang -### -S -ftree-vectorize -fno-vectorize %s 2>&1 | FileCheck -check-prefix=CHECK-NO-VECTORIZE %s
-// RUN: %clang -### -S -O %s 2>&1 | FileCheck -check-prefix=CHECK-VECTORIZE %s
+// RUN: %clang -### -S -O %s 2>&1 | FileCheck -check-prefix=CHECK-NO-VECTORIZE %s
 // RUN: %clang -### -S -O2 %s 2>&1 | FileCheck -check-prefix=CHECK-VECTORIZE %s
 // RUN: %clang -### -S -Os %s 2>&1 | FileCheck -check-prefix=CHECK-VECTORIZE %s
 // RUN: %clang -### -S -O3 %s 2>&1 | FileCheck -check-prefix=CHECK-VECTORIZE %s
@@ -157,7 +157,7 @@
 // RUN: %clang -### -S -fno-tree-slp-vectorize -fslp-vectorize %s 2>&1 | FileCheck -check-prefix=CHECK-SLP-VECTORIZE %s
 // RUN: %clang -### -S -fno-tree-slp-vectorize %s 2>&1 | FileCheck -check-prefix=CHECK-NO-SLP-VECTORIZE %s
 // RUN: %clang -### -S -ftree-slp-vectorize -fno-slp-vectorize %s 2>&1 | FileCheck -check-prefix=CHECK-NO-SLP-VECTORIZE %s
-// RUN: %clang -### -S -O %s 2>&1 | FileCheck -check-prefix=CHECK-SLP-VECTORIZE %s
+// RUN: %clang -### -S -O %s 2>&1 | FileCheck -check-prefix=CHECK-NO-SLP-VECTORIZE %s
 // RUN: %clang -### -S -O2 %s 2>&1 | FileCheck -check-prefix=CHECK-SLP-VECTORIZE %s
 // RUN: %clang -### -S -Os %s 2>&1 | FileCheck -check-prefix=CHECK-SLP-VECTORIZE %s
 // RUN: %clang -### -S -Oz %s 2>&1 | FileCheck -check-prefix=CHECK-SLP-VECTORIZE %s