diff --git a/core/fxcrt/cfx_datetime.cpp b/core/fxcrt/cfx_datetime.cpp
index bedddfd..b06597a 100644
--- a/core/fxcrt/cfx_datetime.cpp
+++ b/core/fxcrt/cfx_datetime.cpp
@@ -10,7 +10,7 @@
 #include "core/fxcrt/fx_system.h"
 #include "third_party/base/check.h"
 
-#if defined(OS_ANDROID) || defined(OS_LINUX) || defined(OS_CHROMEOS) || \
+#if BUILDFLAG(IS_ANDROID) || BUILDFLAG(IS_LINUX) || BUILDFLAG(IS_CHROMEOS) || \
     BUILDFLAG(IS_APPLE) || defined(OS_ASMJS)
 #include <sys/time.h>
 #include <time.h>
diff --git a/core/fxcrt/cfx_fileaccess_posix.h b/core/fxcrt/cfx_fileaccess_posix.h
index a2fc9bf..b752691 100644
--- a/core/fxcrt/cfx_fileaccess_posix.h
+++ b/core/fxcrt/cfx_fileaccess_posix.h
@@ -14,7 +14,7 @@
 #include "core/fxcrt/fileaccess_iface.h"
 #include "core/fxcrt/fx_types.h"
 
-#if !defined(OS_POSIX)
+#if !BUILDFLAG(IS_POSIX)
 #error "Included on the wrong platform"
 #endif
 
diff --git a/core/fxge/cfx_fontmapper.cpp b/core/fxge/cfx_fontmapper.cpp
index 35bd091..61347df 100644
--- a/core/fxge/cfx_fontmapper.cpp
+++ b/core/fxge/cfx_fontmapper.cpp
@@ -155,13 +155,13 @@
     {"ForteMT", "Forte"},
 };
 
-#if defined(OS_LINUX) || defined(OS_CHROMEOS) || defined(OS_ASMJS)
+#if BUILDFLAG(IS_LINUX) || BUILDFLAG(IS_CHROMEOS) || defined(OS_ASMJS)
 const char kNarrowFamily[] = "LiberationSansNarrow";
-#elif defined(OS_ANDROID)
+#elif BUILDFLAG(IS_ANDROID)
 const char kNarrowFamily[] = "RobotoCondensed";
 #else
 const char kNarrowFamily[] = "ArialNarrow";
-#endif  // defined(OS_LINUX) || defined(OS_CHROMEOS) || defined(OS_ASMJS)
+#endif  // BUILDFLAG(IS_LINUX) || BUILDFLAG(IS_CHROMEOS) || defined(OS_ASMJS)
 
 ByteString TT_NormalizeName(ByteString norm) {
   norm.Remove(' ');
@@ -604,7 +604,7 @@
       }
     } else {
       if (Charset == FX_Charset::kSymbol) {
-#if BUILDFLAG(IS_APPLE) || defined(OS_ANDROID)
+#if BUILDFLAG(IS_APPLE) || BUILDFLAG(IS_ANDROID)
         if (SubstName == "Symbol") {
           pSubstFont->m_Family = "Chrome Symbol";
           pSubstFont->m_Charset = FX_Charset::kSymbol;
diff --git a/core/fxge/linux/fx_linux_impl.cpp b/core/fxge/linux/fx_linux_impl.cpp
index beb941a..b5e4793 100644
--- a/core/fxge/linux/fx_linux_impl.cpp
+++ b/core/fxge/linux/fx_linux_impl.cpp
@@ -17,7 +17,7 @@
 #include "third_party/base/check.h"
 #include "third_party/base/cxx17_backports.h"
 
-#if !defined(OS_LINUX) && !defined(OS_CHROMEOS) && !defined(OS_ASMJS)
+#if !BUILDFLAG(IS_LINUX) && !BUILDFLAG(IS_CHROMEOS) && !defined(OS_ASMJS)
 #error "Included on the wrong platform"
 #endif
 
diff --git a/fpdfsdk/fpdf_edit_embeddertest.cpp b/fpdfsdk/fpdf_edit_embeddertest.cpp
index d49633fa..de213c7 100644
--- a/fpdfsdk/fpdf_edit_embeddertest.cpp
+++ b/fpdfsdk/fpdf_edit_embeddertest.cpp
@@ -3043,7 +3043,7 @@
 }
 
 // TODO(npm): Add tests using Japanese fonts in other OS.
-#if defined(OS_LINUX) || defined(OS_CHROMEOS)
+#if BUILDFLAG(IS_LINUX) || BUILDFLAG(IS_CHROMEOS)
 TEST_F(FPDFEditEmbedderTest, AddCIDFontText) {
   // Start with a blank page
   FPDF_PAGE page = FPDFPage_New(CreateNewDocument(), 0, 612, 792);
@@ -3101,7 +3101,7 @@
 
   VerifySavedDocument(612, 792, md5);
 }
-#endif  // defined(OS_LINUX) || defined(OS_CHROMEOS)
+#endif  // BUILDFLAG(IS_LINUX) || BUILDFLAG(IS_CHROMEOS)
 
 // TODO(crbug.com/pdfium/1651): Fix this issue and enable the test for Skia.
 #if defined(_SKIA_SUPPORT_)
diff --git a/fpdfsdk/fpdf_transformpage_embeddertest.cpp b/fpdfsdk/fpdf_transformpage_embeddertest.cpp
index 3f57d74..925e0b5 100644
--- a/fpdfsdk/fpdf_transformpage_embeddertest.cpp
+++ b/fpdfsdk/fpdf_transformpage_embeddertest.cpp
@@ -8,7 +8,7 @@
 #include "testing/embedder_test.h"
 #include "testing/embedder_test_constants.h"
 
-#if defined(OS_LINUX) || defined(OS_CHROMEOS) || defined(OS_FUCHSIA)
+#if BUILDFLAG(IS_LINUX) || BUILDFLAG(IS_CHROMEOS) || BUILDFLAG(IS_FUCHSIA)
 #include "third_party/base/test/scoped_locale.h"
 #endif
 
@@ -473,7 +473,7 @@
   }
 }
 
-#if defined(OS_LINUX) || defined(OS_CHROMEOS) || defined(OS_FUCHSIA)
+#if BUILDFLAG(IS_LINUX) || BUILDFLAG(IS_CHROMEOS) || BUILDFLAG(IS_FUCHSIA)
 TEST_F(FPDFTransformEmbedderTest, TransFormWithClipAndSaveWithLocale) {
   pdfium::base::ScopedLocale scoped_locale("da_DK.UTF-8");
 
@@ -529,4 +529,5 @@
     CloseSavedDocument();
   }
 }
-#endif  // defined(OS_LINUX) || defined(OS_CHROMEOS) || defined(OS_FUCHSIA)
+#endif  // BUILDFLAG(IS_LINUX) || BUILDFLAG(IS_CHROMEOS) ||
+        // BUILDFLAG(IS_FUCHSIA)
diff --git a/fxjs/cjs_publicmethods.cpp b/fxjs/cjs_publicmethods.cpp
index a13b95d..39b3b82 100644
--- a/fxjs/cjs_publicmethods.cpp
+++ b/fxjs/cjs_publicmethods.cpp
@@ -67,7 +67,7 @@
 
 namespace {
 
-#if !defined(OS_ANDROID)
+#if !BUILDFLAG(IS_ANDROID)
 constexpr double kDoubleCorrect = 0.000000000000001;
 #endif
 
@@ -106,7 +106,7 @@
   }
 }
 
-#if !defined(OS_ANDROID)
+#if !BUILDFLAG(IS_ANDROID)
 ByteString CalculateString(double dValue,
                            int iDec,
                            int* iDec2,
@@ -179,7 +179,7 @@
   return c == '.' || c == ',';
 }
 
-#if !defined(OS_ANDROID)
+#if !BUILDFLAG(IS_ANDROID)
 bool IsStyleWithDigitSeparator(int style) {
   return style == 0 || style == 2;
 }
@@ -202,7 +202,7 @@
   return IsStyleWithCommaDecimalMark(style) ? ',' : '.';
 }
 
-#if !defined(OS_ANDROID)
+#if !BUILDFLAG(IS_ANDROID)
 void NormalizeDecimalMark(ByteString* str) {
   str->Replace(",", ".");
 }
@@ -593,7 +593,7 @@
 CJS_Result CJS_PublicMethods::AFNumber_Format(
     CJS_Runtime* pRuntime,
     const std::vector<v8::Local<v8::Value>>& params) {
-#if !defined(OS_ANDROID)
+#if !BUILDFLAG(IS_ANDROID)
   if (params.size() != 6)
     return CJS_Result::Failure(JSMessage::kParamError);
 
@@ -787,7 +787,7 @@
 CJS_Result CJS_PublicMethods::AFPercent_Format(
     CJS_Runtime* pRuntime,
     const std::vector<v8::Local<v8::Value>>& params) {
-#if !defined(OS_ANDROID)
+#if !BUILDFLAG(IS_ANDROID)
   if (params.size() < 2)
     return CJS_Result::Failure(JSMessage::kParamError);
 
diff --git a/fxjs/cjs_util.cpp b/fxjs/cjs_util.cpp
index 91aacab..5823ee1 100644
--- a/fxjs/cjs_util.cpp
+++ b/fxjs/cjs_util.cpp
@@ -27,7 +27,7 @@
 #include "third_party/base/cxx17_backports.h"
 #include "v8/include/v8-date.h"
 
-#if defined(OS_ANDROID)
+#if BUILDFLAG(IS_ANDROID)
 #include <ctype.h>
 #endif
 
diff --git a/third_party/base/allocator/partition_allocator/address_space_randomization.h b/third_party/base/allocator/partition_allocator/address_space_randomization.h
index 620028c..7ab4cb2 100644
--- a/third_party/base/allocator/partition_allocator/address_space_randomization.h
+++ b/third_party/base/allocator/partition_allocator/address_space_randomization.h
@@ -94,7 +94,7 @@
       return AslrAddress(0x1000000000ULL);
     }
 
-  #elif defined(OS_POSIX) || defined(OS_FUCHSIA)
+  #elif BUILDFLAG(IS_POSIX) || BUILDFLAG(IS_FUCHSIA)
 
     #if defined(ARCH_CPU_X86_64)
 
@@ -109,7 +109,7 @@
 
     #elif defined(ARCH_CPU_ARM64)
 
-      #if defined(OS_ANDROID)
+      #if BUILDFLAG(IS_ANDROID)
 
       // Restrict the address range on Android to avoid a large performance
       // regression in single-process WebViews. See https://crbug.com/837640.
@@ -238,7 +238,7 @@
     #endif  // !defined(ARCH_CPU_X86_64) && !defined(ARCH_CPU_PPC64) &&
             // !defined(ARCH_CPU_S390X) && !defined(ARCH_CPU_S390)
 
-  #endif  // defined(OS_POSIX)
+  #endif  // BUILDFLAG(IS_POSIX)
 
 #elif defined(ARCH_CPU_32_BITS)
 
diff --git a/third_party/base/allocator/partition_allocator/page_allocator.cc b/third_party/base/allocator/partition_allocator/page_allocator.cc
index dad3cf1..32094f9 100644
--- a/third_party/base/allocator/partition_allocator/page_allocator.cc
+++ b/third_party/base/allocator/partition_allocator/page_allocator.cc
@@ -22,7 +22,7 @@
 
 #if BUILDFLAG(IS_WIN)
 #include "third_party/base/allocator/partition_allocator/page_allocator_internals_win.h"
-#elif defined(OS_POSIX) || defined(OS_FUCHSIA)
+#elif BUILDFLAG(IS_POSIX) || BUILDFLAG(IS_FUCHSIA)
 #include "third_party/base/allocator/partition_allocator/page_allocator_internals_posix.h"
 #else
 #error Platform not supported.
diff --git a/third_party/base/allocator/partition_allocator/page_allocator_internals_posix.h b/third_party/base/allocator/partition_allocator/page_allocator_internals_posix.h
index 646cbb7..23bcc76 100644
--- a/third_party/base/allocator/partition_allocator/page_allocator_internals_posix.h
+++ b/third_party/base/allocator/partition_allocator/page_allocator_internals_posix.h
@@ -13,10 +13,10 @@
 #if BUILDFLAG(IS_APPLE)
 #include <mach/mach.h>
 #endif
-#if defined(OS_ANDROID)
+#if BUILDFLAG(IS_ANDROID)
 #include <sys/prctl.h>
 #endif
-#if defined(OS_LINUX) || defined(OS_CHROMEOS)
+#if BUILDFLAG(IS_LINUX) || BUILDFLAG(IS_CHROMEOS)
 #include <sys/resource.h>
 
 #include <algorithm>
@@ -33,7 +33,7 @@
 namespace pdfium {
 namespace base {
 
-#if defined(OS_ANDROID)
+#if BUILDFLAG(IS_ANDROID)
 namespace {
 const char* PageTagToName(PageTag tag) {
   // Important: All the names should be string literals. As per prctl.h in
@@ -57,7 +57,7 @@
   }
 }
 }  // namespace
-#endif  // defined(OS_ANDROID)
+#endif  // BUILDFLAG(IS_ANDROID)
 
 // |mmap| uses a nearby address if the hint address is blocked.
 constexpr bool kHintIsAdvisory = true;
@@ -101,7 +101,7 @@
 
   // TODO(https://crbug.com/927411): Remove once Fuchsia uses a native page
   // allocator, rather than relying on POSIX compatibility.
-#if defined(OS_FUCHSIA)
+#if BUILDFLAG(IS_FUCHSIA)
   if (page_tag == PageTag::kV8) {
     map_flags |= MAP_JIT;
   }
@@ -113,7 +113,7 @@
     ret = nullptr;
   }
 
-#if defined(OS_ANDROID)
+#if BUILDFLAG(IS_ANDROID)
   // On Android, anonymous mappings can have a name attached to them. This is
   // useful for debugging, and double-checking memory attribution.
   if (ret) {
diff --git a/third_party/base/allocator/partition_allocator/spin_lock.cc b/third_party/base/allocator/partition_allocator/spin_lock.cc
index bb9c5ac..d7cdd15 100644
--- a/third_party/base/allocator/partition_allocator/spin_lock.cc
+++ b/third_party/base/allocator/partition_allocator/spin_lock.cc
@@ -9,7 +9,7 @@
 
 #if BUILDFLAG(IS_WIN)
 #include <windows.h>
-#elif defined(OS_POSIX) || defined(OS_FUCHSIA)
+#elif BUILDFLAG(IS_POSIX) || BUILDFLAG(IS_FUCHSIA)
 #include <sched.h>
 #endif
 
@@ -28,7 +28,7 @@
 #define YIELD_PROCESSOR YieldProcessor()
 #define YIELD_THREAD SwitchToThread()
 
-#elif defined(OS_POSIX) || defined(OS_FUCHSIA)
+#elif BUILDFLAG(IS_POSIX) || BUILDFLAG(IS_FUCHSIA)
 
 #if defined(ARCH_CPU_X86_64) || defined(ARCH_CPU_X86)
 #define YIELD_PROCESSOR __asm__ __volatile__("pause")
diff --git a/third_party/base/compiler_specific.h b/third_party/base/compiler_specific.h
index 6672f7e..e5ce291 100644
--- a/third_party/base/compiler_specific.h
+++ b/third_party/base/compiler_specific.h
@@ -111,7 +111,7 @@
 #endif
 
 // MemorySanitizer annotations.
-#if defined(MEMORY_SANITIZER) && !defined(OS_NACL)
+#if defined(MEMORY_SANITIZER) && !BUILDFLAG(IS_NACL)
 #include <sanitizer/msan_interface.h>
 
 // Mark a memory region fully initialized.
diff --git a/third_party/base/immediate_crash.h b/third_party/base/immediate_crash.h
index fd2fd05..658b381 100644
--- a/third_party/base/immediate_crash.h
+++ b/third_party/base/immediate_crash.h
@@ -41,7 +41,7 @@
 
 #if defined(COMPILER_GCC)
 
-#if defined(OS_NACL)
+#if BUILDFLAG(IS_NACL)
 
 // Crash report accuracy is not guaranteed on NaCl.
 #define TRAP_SEQUENCE1_() __builtin_trap()
diff --git a/third_party/base/memory/aligned_memory.cc b/third_party/base/memory/aligned_memory.cc
index 8fe2f8a..10884e8 100644
--- a/third_party/base/memory/aligned_memory.cc
+++ b/third_party/base/memory/aligned_memory.cc
@@ -7,7 +7,7 @@
 #include "build/build_config.h"
 #include "third_party/base/check_op.h"
 
-#if defined(OS_ANDROID)
+#if BUILDFLAG(IS_ANDROID)
 #include <malloc.h>
 #endif
 
@@ -21,7 +21,7 @@
   void* ptr = nullptr;
 #if defined(COMPILER_MSVC)
   ptr = _aligned_malloc(size, alignment);
-#elif defined(OS_ANDROID)
+#elif BUILDFLAG(IS_ANDROID)
   // Android technically supports posix_memalign(), but does not expose it in
   // the current version of the library headers used by Chrome.  Luckily,
   // memalign() on Android returns pointers which can safely be used with
diff --git a/xfa/fgas/font/fgas_fontutils.cpp b/xfa/fgas/font/fgas_fontutils.cpp
index 6b0e8a7..8ebcffe 100644
--- a/xfa/fgas/font/fgas_fontutils.cpp
+++ b/xfa/fgas/font/fgas_fontutils.cpp
@@ -1103,7 +1103,7 @@
     {0xfef135f8, "AdobeHeitiStd-Regular", "Heiti TC,STHeiti", 0,
      FX_CodePage::kChineseSimplified},
 };
-#elif defined(OS_ANDROID)
+#elif BUILDFLAG(IS_ANDROID)
 const FGAS_FontInfo kXFAFontsMap[] = {
     {0x01d5d33e, "SimSun", "Droid Sans Fallback", 0,
      FX_CodePage::kChineseSimplified},
@@ -1747,7 +1747,7 @@
     {0xfef135f8, "AdobeHeitiStd-Regular", "Droid Sans Fallback", 0,
      FX_CodePage::kChineseSimplified},
 };
-#elif defined(OS_POSIX)
+#elif BUILDFLAG(IS_POSIX)
 const FGAS_FontInfo kXFAFontsMap[] = {
     {0x01d5d33e, "SimSun",
      "WenQuanYi Zen Hei Mono,AR PL UMing CN,AR PL UMing HK,AR PL UMing TW,AR "
diff --git a/xfa/fwl/cfwl_edit_embeddertest.cpp b/xfa/fwl/cfwl_edit_embeddertest.cpp
index 27679a3..59254c4 100644
--- a/xfa/fwl/cfwl_edit_embeddertest.cpp
+++ b/xfa/fwl/cfwl_edit_embeddertest.cpp
@@ -162,11 +162,11 @@
   // abcde
   // fghij|
   {
-#if defined(OS_LINUX) || defined(OS_CHROMEOS)
+#if BUILDFLAG(IS_LINUX) || BUILDFLAG(IS_CHROMEOS)
     const char kFilledMultilineMD5[] = "fc1f4d5fdb2c5755005fc525b0a60ec9";
 #else
     const char kFilledMultilineMD5[] = "a5654e027d8b1667c20f3b86d1918003";
-#endif  // defined(OS_LINUX) || defined(OS_CHROMEOS)
+#endif  // BUILDFLAG(IS_LINUX) || BUILDFLAG(IS_CHROMEOS)
     ScopedFPDFBitmap page_bitmap =
         RenderLoadedPageWithFlags(page(), FPDF_ANNOT);
     CompareBitmap(page_bitmap.get(), 612, 792, kFilledMultilineMD5);
@@ -187,11 +187,11 @@
   // Should look like:
   // abcde|ghij
   {
-#if defined(OS_LINUX) || defined(OS_CHROMEOS)
+#if BUILDFLAG(IS_LINUX) || BUILDFLAG(IS_CHROMEOS)
     const char kMultilineBackspaceMD5[] = "8bb62a8100ff1e1cc113d4033e0d824e";
 #else
     const char kMultilineBackspaceMD5[] = "a2f1dcab92bb1fb7c2f9ccc70100c989";
-#endif  // defined(OS_LINUX) || defined(OS_CHROMEOS)
+#endif  // BUILDFLAG(IS_LINUX) || BUILDFLAG(IS_CHROMEOS)
     ScopedFPDFBitmap page_bitmap =
         RenderLoadedPageWithFlags(page(), FPDF_ANNOT);
     CompareBitmap(page_bitmap.get(), 612, 792, kMultilineBackspaceMD5);