diff --git a/deps/oblib/src/rpc/frame/ob_req_packet_code.h b/deps/oblib/src/rpc/frame/ob_req_packet_code.h
index 2c7d89834..86d422ebf 100644
--- a/deps/oblib/src/rpc/frame/ob_req_packet_code.h
+++ b/deps/oblib/src/rpc/frame/ob_req_packet_code.h
@@ -90,6 +90,11 @@ enum ObReqPacketCode : int32_t
 // obcall transport header so the config path keeps compiling. The accessor names
 // keep the rpc_checksum wording because they mirror the _rpc_checksum config.
 // ---------------------------------------------------------------------------
+// `OPTIONAL` is an (empty) macro in the Windows SDK (winnt.h). Undefine it so
+// the enumerator below is not eaten by the preprocessor on MSVC/clang-cl.
+#ifdef OPTIONAL
+#undef OPTIONAL
+#endif
 enum class ObReqCheckSumCheckLevel
 {
   INVALID,
diff --git a/src/observer/CMakeLists.txt b/src/observer/CMakeLists.txt
index 8a8ae594b..5631970dd 100644
--- a/src/observer/CMakeLists.txt
+++ b/src/observer/CMakeLists.txt
@@ -521,9 +521,7 @@ if(WIN32)
   endif()
   # oblib_add_extra_objects creates separate .lib files not merged into oblib.lib on Windows
   target_link_libraries(observer_without_bolt PRIVATE
-    ${CMAKE_BINARY_DIR}/deps/oblib/src/lib/compress/zstd_1_3_8/zstd_1_3_8_objs.lib
-    ${CMAKE_BINARY_DIR}/deps/oblib/src/lib/compress/zstd/zstd_objs.lib
-    ${CMAKE_BINARY_DIR}/deps/oblib/src/lib/compress/lz4/lz4-all.lib)
+    ${CMAKE_BINARY_DIR}/deps/oblib/src/lib/compress/zstd_1_3_8/zstd_1_3_8_objs.lib)
   # Itanium ABI _Unwind_* stubs (Windows uses SEH; PL unwinding non-functional)
   target_sources(observer_without_bolt PRIVATE ${CMAKE_CURRENT_SOURCE_DIR}/win32_unwind_stubs.c)
 endif()