diff --git a/lld/ELF/Config.h b/lld/ELF/Config.h
--- a/lld/ELF/Config.h
+++ b/lld/ELF/Config.h
@@ -251,7 +251,8 @@
   uint64_t mipsGotSize;
   uint64_t zStackSize;
   unsigned ltoPartitions;
-  unsigned ltoo;
+  unsigned ltoOptLevel;
+  unsigned ltoSizeLevel;
   unsigned optimize;
   StringRef thinLTOJobs;
   unsigned timeTraceGranularity;
diff --git a/lld/ELF/Driver.cpp b/lld/ELF/Driver.cpp
--- a/lld/ELF/Driver.cpp
+++ b/lld/ELF/Driver.cpp
@@ -833,6 +833,38 @@
   return ret;
 }
 
+static unsigned getOptLevel(opt::InputArgList &Args) {
+  if (opt::Arg *A = Args.getLastArg(OPT_lto_O)) {
+    StringRef S = A->getValue();
+    if (S == "s" || S == "z" || S.empty())
+      return 2;
+    if (S == "g")
+      return 1;
+    int V;
+    if (!to_integer(A->getValue(), V)) {
+      StringRef Spelling = Args.getArgString(A->getIndex());
+      error(Spelling + ": number expected, but got '" + S + "'");
+      return 0;
+    }
+    return V;
+  }
+  return 2;
+}
+
+static unsigned getSizeLevel(opt::InputArgList &Args) {
+  if (opt::Arg *A = Args.getLastArg(OPT_lto_O)) {
+    switch (A->getValue()[0]) {
+    default:
+      return 0;
+    case 's':
+      return 1;
+    case 'z':
+      return 2;
+    }
+  }
+  return 0;
+}
+
 // Parse the symbol ordering file and warn for any duplicate entries.
 static std::vector<StringRef> getSymbolOrderingFile(MemoryBufferRef mb) {
   SetVector<StringRef> names;
@@ -924,7 +956,8 @@
   config->ltoNewPmPasses = args.getLastArgValue(OPT_lto_newpm_passes);
   config->ltoWholeProgramVisibility =
       args.hasArg(OPT_lto_whole_program_visibility);
-  config->ltoo = args::getInteger(args, OPT_lto_O, 2);
+  config->ltoOptLevel = getOptLevel(args);
+  config->ltoSizeLevel = getSizeLevel(args);
   config->ltoObjPath = args.getLastArgValue(OPT_lto_obj_path_eq);
   config->ltoPartitions = args::getInteger(args, OPT_lto_partitions, 1);
   config->ltoSampleProfile = args.getLastArgValue(OPT_lto_sample_profile);
@@ -1066,8 +1099,10 @@
   if (auto *arg = args.getLastArg(OPT_thinlto_jobs))
     config->thinLTOJobs = arg->getValue();
 
-  if (config->ltoo > 3)
-    error("invalid optimization level for LTO: " + Twine(config->ltoo));
+  if (config->ltoOptLevel > 3)
+    error("invalid optimization level for LTO: " + Twine(config->ltoOptLevel));
+  if (config->ltoSizeLevel > 2)
+    error("invalid size level for LTO: " + Twine(config->ltoSizeLevel));
   if (config->ltoPartitions == 0)
     error("--lto-partitions: number of threads must be > 0");
   if (!get_threadpool_strategy(config->thinLTOJobs))
diff --git a/lld/ELF/LTO.cpp b/lld/ELF/LTO.cpp
--- a/lld/ELF/LTO.cpp
+++ b/lld/ELF/LTO.cpp
@@ -114,10 +114,11 @@
   c.CodeModel = getCodeModelFromCMModel();
   c.DisableVerify = config->disableVerify;
   c.DiagHandler = diagnosticHandler;
-  c.OptLevel = config->ltoo;
+  c.OptLevel = config->ltoOptLevel;
+  c.SizeLevel = config->ltoSizeLevel;
   c.CPU = getCPUStr();
   c.MAttrs = getMAttrs();
-  c.CGOptLevel = args::getCGOptLevel(config->ltoo);
+  c.CGOptLevel = args::getCGOptLevel(config->ltoOptLevel);
 
   c.PTO.LoopVectorization = c.OptLevel > 1;
   c.PTO.SLPVectorization = c.OptLevel > 1;
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
@@ -8,8 +8,16 @@
 ; RUN: llvm-nm %t2 | 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
+; RUN: ld.lld -o %t2 -e main %t.o --plugin-opt=O2
 ; RUN: llvm-nm %t2 | FileCheck --check-prefix=CHECK-O2 %s
+; RUN: ld.lld -o %t0 -e main --lto-Oz %t.o
+; RUN: llvm-nm %t0 | FileCheck --check-prefix=CHECK-OZ %s
+; RUN: ld.lld -o %tz -e main %t.o --plugin-opt=Oz
+; RUN: llvm-nm %tz | FileCheck --check-prefix=CHECK-OZ %s
+; RUN: ld.lld -o %t0 -e main --lto-Os %t.o
+; RUN: llvm-nm %t0 | FileCheck --check-prefix=CHECK-OS %s
+; RUN: ld.lld -o %tz -e main %t.o --plugin-opt=Os
+; RUN: llvm-nm %tz | FileCheck --check-prefix=CHECK-OS %s
 
 ; Reject invalid optimization levels.
 ; RUN: not ld.lld -o /dev/null -e main --lto-O6 %t.o 2>&1 | \
@@ -33,6 +41,8 @@
 
 ; CHECK-O0: foo
 ; CHECK-O2-NOT: foo
+; CHECK-OZ-NOT: foo
+; CHECK-OS-NOT: foo
 define internal void @foo() {
   ret void
 }
diff --git a/llvm/include/llvm/LTO/Config.h b/llvm/include/llvm/LTO/Config.h
--- a/llvm/include/llvm/LTO/Config.h
+++ b/llvm/include/llvm/LTO/Config.h
@@ -47,6 +47,7 @@
   CodeGenOpt::Level CGOptLevel = CodeGenOpt::Default;
   CodeGenFileType CGFileType = CGFT_ObjectFile;
   unsigned OptLevel = 2;
+  unsigned SizeLevel = 0;
   bool DisableVerify = false;
 
   /// Use the new pass manager
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
@@ -180,8 +180,7 @@
 }
 
 static void runNewPMPasses(const Config &Conf, Module &Mod, TargetMachine *TM,
-                           unsigned OptLevel, bool IsThinLTO,
-                           ModuleSummaryIndex *ExportSummary,
+                           bool IsThinLTO, ModuleSummaryIndex *ExportSummary,
                            const ModuleSummaryIndex *ImportSummary) {
   Optional<PGOOptions> PGOOpt;
   if (!Conf.SampleProfile.empty())
@@ -227,7 +226,7 @@
 
   PassBuilder::OptimizationLevel OL;
 
-  switch (OptLevel) {
+  switch (Conf.OptLevel) {
   default:
     llvm_unreachable("Invalid optimization level");
   case 0:
@@ -238,6 +237,19 @@
     break;
   case 2:
     OL = PassBuilder::OptimizationLevel::O2;
+    switch (Conf.SizeLevel) {
+    default:
+      llvm_unreachable("Invalid optimization level for size");
+    case 0:
+      OL = PassBuilder::OptimizationLevel::O2;
+      break;
+    case 1:
+      OL = PassBuilder::OptimizationLevel::Os;
+      break;
+    case 2:
+      OL = PassBuilder::OptimizationLevel::Oz;
+      break;
+    }
     break;
   case 3:
     OL = PassBuilder::OptimizationLevel::O3;
@@ -317,6 +329,7 @@
   PMB.LoopVectorize = true;
   PMB.SLPVectorize = true;
   PMB.OptLevel = Conf.OptLevel;
+  PMB.SizeLevel = Conf.SizeLevel;
   PMB.PGOSampleUse = Conf.SampleProfile;
   PMB.EnablePGOCSInstrGen = Conf.RunCSIRInstr;
   if (!Conf.RunCSIRInstr && !Conf.CSIRProfile.empty()) {
@@ -338,8 +351,7 @@
     runNewPMCustomPasses(Conf, Mod, TM, Conf.OptPipeline, Conf.AAPipeline,
                          Conf.DisableVerify);
   else if (Conf.UseNewPM)
-    runNewPMPasses(Conf, Mod, TM, Conf.OptLevel, IsThinLTO, ExportSummary,
-                   ImportSummary);
+    runNewPMPasses(Conf, Mod, TM, IsThinLTO, ExportSummary, ImportSummary);
   else
     runOldPMPasses(Conf, Mod, TM, IsThinLTO, ExportSummary, ImportSummary);
   return !Conf.PostOptModuleHook || Conf.PostOptModuleHook(Task, Mod);