diff --git a/lld/test/COFF/lto-opt-level.ll b/lld/test/COFF/lto-opt-level.ll
--- a/lld/test/COFF/lto-opt-level.ll
+++ b/lld/test/COFF/lto-opt-level.ll
@@ -4,6 +4,10 @@
 ; RUN: FileCheck --check-prefix=CHECK-O0 %s < %t0.map
 ; RUN: lld-link /out:%t2.exe /entry:main /subsystem:console /opt:lldlto=2 /lldmap:%t2.map %t.obj
 ; RUN: FileCheck --check-prefix=CHECK-O2 %s < %t2.map
+; RUN: lld-link /out:%ts.exe /entry:main /subsystem:console /opt:lldlto=s /lldmap:%ts.map %t.obj
+; RUN: FileCheck --check-prefix=CHECK-O2 %s < %ts.map
+; RUN: lld-link /out:%tz.exe /entry:main /subsystem:console /opt:lldlto=z /lldmap:%tz.map %t.obj
+; RUN: FileCheck --check-prefix=CHECK-O2 %s < %tz.map
 ; RUN: lld-link /out:%t2a.exe /entry:main /subsystem:console /lldmap:%t2a.map %t.obj
 ; RUN: FileCheck --check-prefix=CHECK-O2 %s < %t2a.map
 
diff --git a/lld/test/ELF/lto/opt-level.ll b/lld/test/ELF/lto/opt-level.ll
--- a/lld/test/ELF/lto/opt-level.ll
+++ b/lld/test/ELF/lto/opt-level.ll
@@ -6,6 +6,10 @@
 ; RUN: llvm-nm %t0 | FileCheck --check-prefix=CHECK-O0 %s
 ; RUN: ld.lld -o %t2 -e main --lto-O2 %t.o
 ; RUN: llvm-nm %t2 | FileCheck --check-prefix=CHECK-O2 %s
+; RUN: ld.lld -o %ts -e main --lto-Os %t.o
+; RUN: llvm-nm %ts | FileCheck --check-prefix=CHECK-O2 %s
+; RUN: ld.lld -o %tz -e main --lto-Oz %t.o
+; RUN: llvm-nm %tz | FileCheck --check-prefix=CHECK-O2 %s
 ; RUN: ld.lld -o %t2a -e main %t.o
 ; RUN: llvm-nm %t2a | FileCheck --check-prefix=CHECK-O2 %s
 ; RUN: ld.lld -o %t2 -e main %t.o --plugin-opt O2
@@ -19,14 +23,14 @@
 ; RUN:   FileCheck --check-prefix=INVALID1 %s
 ; RUN: not ld.lld -o %t3 -e main --plugin-opt=Ofoo %t.o 2>&1 | \
 ; RUN:   FileCheck --check-prefix=INVALID2 %s
-; INVALID2: --plugin-opt=Ofoo: number expected, but got 'foo'
+; INVALID2: invalid optimization level for LTO: foo
 
 ; RUN: not ld.lld -o %t3 -e main --lto-O-1 %t.o 2>&1 | \
 ; RUN:   FileCheck --check-prefix=INVALIDNEGATIVE1 %s
-; INVALIDNEGATIVE1: invalid optimization level for LTO: 4294967295
+; INVALIDNEGATIVE1: invalid optimization level for LTO: -1
 ; RUN: not ld.lld -o %t3 -e main --plugin-opt=O-1 %t.o 2>&1 | \
 ; RUN:   FileCheck --check-prefix=INVALIDNEGATIVE2 %s
-; INVALIDNEGATIVE2: invalid optimization level for LTO: 4294967295
+; INVALIDNEGATIVE2: invalid optimization level for LTO: -1
 
 target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
 target triple = "x86_64-unknown-linux-gnu"
diff --git a/lld/test/wasm/lto/opt-level.ll b/lld/test/wasm/lto/opt-level.ll
--- a/lld/test/wasm/lto/opt-level.ll
+++ b/lld/test/wasm/lto/opt-level.ll
@@ -3,6 +3,10 @@
 ; RUN: obj2yaml %t0 | FileCheck --check-prefix=CHECK-O0 %s
 ; RUN: wasm-ld -o %t2 -e main --lto-O2 %t.o
 ; RUN: obj2yaml %t2 | FileCheck --check-prefix=CHECK-O2 %s
+; RUN: wasm-ld -o %ts -e main --lto-Os %t.o
+; RUN: obj2yaml %ts | FileCheck --check-prefix=CHECK-O2 %s
+; RUN: wasm-ld -o %tz -e main --lto-Oz %t.o
+; RUN: obj2yaml %tz | FileCheck --check-prefix=CHECK-O2 %s
 ; RUN: wasm-ld -o %t2a -e main %t.o
 ; RUN: obj2yaml %t2a | FileCheck --check-prefix=CHECK-O2 %s
 
@@ -13,7 +17,7 @@
 
 ; RUN: not wasm-ld -o %t3 -m elf_x86_64 -e main --lto-O-1 %t.o 2>&1 | \
 ; RUN:   FileCheck --check-prefix=INVALIDNEGATIVE %s
-; INVALIDNEGATIVE: invalid optimization level for LTO: 4294967295
+; INVALIDNEGATIVE: invalid optimization level for LTO: -1
 
 target datalayout = "e-m:e-p:32:32-i64:64-n32:64-S128"
 target triple = "wasm32-unknown-unknown-wasm"
diff --git a/llvm/lib/LTO/LTOBackend.cpp b/llvm/lib/LTO/LTOBackend.cpp
--- a/llvm/lib/LTO/LTOBackend.cpp
+++ b/llvm/lib/LTO/LTOBackend.cpp
@@ -155,6 +155,9 @@
   default:
     llvm_unreachable("Invalid optimization level!");
 
+  case 0:
+    return PassBuilder::O0;
+
   case 1:
     return PassBuilder::O1;
 
@@ -315,8 +318,10 @@
   if (!Conf.OptPipeline.empty())
     runNewPMCustomPasses(Mod, TM, Conf.OptPipeline, Conf.AAPipeline,
                          Conf.DisableVerify);
-  auto runPasses = Conf.UseNewPM ? runNewPMPasses : runOldPMPasses;
-  runPasses(Conf, Mod, TM, IsThinLTO, ExportSummary, ImportSummary);
+  else {
+    auto runPasses = Conf.UseNewPM ? runNewPMPasses : runOldPMPasses;
+    runPasses(Conf, Mod, TM, IsThinLTO, ExportSummary, ImportSummary);
+  }
   return !Conf.PostOptModuleHook || Conf.PostOptModuleHook(Task, Mod);
 }
 
diff --git a/llvm/test/tools/gold/X86/opt-level.ll b/llvm/test/tools/gold/X86/opt-level.ll
--- a/llvm/test/tools/gold/X86/opt-level.ll
+++ b/llvm/test/tools/gold/X86/opt-level.ll
@@ -11,6 +11,14 @@
 ; RUN:    -m elf_x86_64 \
 ; RUN:    -plugin-opt=O2 -r -o %t.o %t.bc
 ; RUN: llvm-dis < %t.o.0.4.opt.bc -o - | FileCheck --check-prefix=CHECK-O2 %s
+; RUN: %gold -plugin %llvmshlibdir/LLVMgold%shlibext -plugin-opt=save-temps \
+; RUN:    -m elf_x86_64 \
+; RUN:    -plugin-opt=Os -r -o %t.o %t.bc
+; RUN: llvm-dis < %t.o.0.4.opt.bc -o - | FileCheck --check-prefix=CHECK-O2 %s
+; RUN: %gold -plugin %llvmshlibdir/LLVMgold%shlibext -plugin-opt=save-temps \
+; RUN:    -m elf_x86_64 \
+; RUN:    -plugin-opt=Oz -r -o %t.o %t.bc
+; RUN: llvm-dis < %t.o.0.4.opt.bc -o - | FileCheck --check-prefix=CHECK-O2 %s
 
 ; RUN: %gold -plugin %llvmshlibdir/LLVMgold%shlibext -plugin-opt=save-temps \
 ; RUN:    -m elf_x86_64 --plugin-opt=new-pass-manager \
@@ -24,6 +32,14 @@
 ; RUN:    -m elf_x86_64 --plugin-opt=new-pass-manager \
 ; RUN:    -plugin-opt=O2 -r -o %t.o %t.bc
 ; RUN: llvm-dis < %t.o.0.4.opt.bc -o - | FileCheck --check-prefix=CHECK-O2 %s
+; RUN: %gold -plugin %llvmshlibdir/LLVMgold%shlibext -plugin-opt=save-temps \
+; RUN:    -m elf_x86_64 --plugin-opt=new-pass-manager \
+; RUN:    -plugin-opt=Os -r -o %t.o %t.bc
+; RUN: llvm-dis < %t.o.0.4.opt.bc -o - | FileCheck --check-prefix=CHECK-O2 %s
+; RUN: %gold -plugin %llvmshlibdir/LLVMgold%shlibext -plugin-opt=save-temps \
+; RUN:    -m elf_x86_64 --plugin-opt=new-pass-manager \
+; RUN:    -plugin-opt=Oz -r -o %t.o %t.bc
+; RUN: llvm-dis < %t.o.0.4.opt.bc -o - | FileCheck --check-prefix=CHECK-O2 %s
 
 ; CHECK-O0: define internal void @foo(
 ; CHECK-O1: define internal void @foo(
diff --git a/llvm/test/tools/lto/opt-level.ll b/llvm/test/tools/lto/opt-level.ll
--- a/llvm/test/tools/lto/opt-level.ll
+++ b/llvm/test/tools/lto/opt-level.ll
@@ -3,6 +3,10 @@
 ; RUN: llvm-nm --no-llvm-bc %t.dylib | FileCheck --check-prefix=CHECK-O0 %s
 ; RUN: %ld64 -lto_library %llvmshlibdir/libLTO.dylib -arch x86_64 -dylib -mllvm -O2 -o %t.dylib %t.o
 ; RUN: llvm-nm --no-llvm-bc %t.dylib | FileCheck --check-prefix=CHECK-O2 %s
+; RUN: %ld64 -lto_library %llvmshlibdir/libLTO.dylib -arch x86_64 -dylib -mllvm -Os -o %t.dylib %t.o
+; RUN: llvm-nm --no-llvm-bc %t.dylib | FileCheck --check-prefix=CHECK-O2 %s
+; RUN: %ld64 -lto_library %llvmshlibdir/libLTO.dylib -arch x86_64 -dylib -mllvm -Oz -o %t.dylib %t.o
+; RUN: llvm-nm --no-llvm-bc %t.dylib | FileCheck --check-prefix=CHECK-O2 %s
 
 target triple = "x86_64-apple-macosx10.8.0"
 
diff --git a/llvm/tools/gold/gold-plugin.cpp b/llvm/tools/gold/gold-plugin.cpp
--- a/llvm/tools/gold/gold-plugin.cpp
+++ b/llvm/tools/gold/gold-plugin.cpp
@@ -131,8 +131,8 @@
     OT_SAVE_TEMPS
   };
   static OutputType TheOutputType = OT_NORMAL;
-  static unsigned OptLevel;
-  static unsigned SizeLevel;
+  static unsigned OptLevel = 2;
+  static unsigned SizeLevel = 0;
   // Default parallelism of 0 used to indicate that user did not specify.
   // Actual parallelism default value depends on implementation.
   // Currently only affects ThinLTO, where the default is
diff --git a/llvm/tools/llvm-lto2/llvm-lto2.cpp b/llvm/tools/llvm-lto2/llvm-lto2.cpp
--- a/llvm/tools/llvm-lto2/llvm-lto2.cpp
+++ b/llvm/tools/llvm-lto2/llvm-lto2.cpp
@@ -29,10 +29,11 @@
 using namespace llvm;
 using namespace lto;
 
-static cl::opt<char>
-    OptLevel("O", cl::desc("Optimization level. [-O0, -O1, -O2, or -O3] "
-                           "(default = '-O2')"),
-             cl::Prefix, cl::ZeroOrMore, cl::init('2'));
+static cl::opt<std::string>
+    OptLevel("O",
+             cl::desc("Optimization level. [-O0, -O1, -O2, -Os, -Oz, or "
+                      "-O3] (default = '-O2')"),
+             cl::Prefix, cl::ZeroOrMore, cl::init("2"));
 
 static cl::opt<char> CGOptLevel(
     "cg-opt-level",
@@ -244,7 +245,10 @@
   Conf.OptPipeline = OptPipeline;
   Conf.AAPipeline = AAPipeline;
 
-  Conf.OptLevel = OptLevel - '0';
+  Conf.OptLevel =
+      check(llvm::lto::getOptLevel(OptLevel), "invalid optimization level");
+  Conf.SizeLevel =
+      check(llvm::lto::getSizeLevel(OptLevel), "invalid optimization level");
   Conf.UseNewPM = UseNewPM;
   switch (CGOptLevel) {
   case '0':