diff --git a/CMakeLists.txt b/CMakeLists.txt
index 5440be22f6..f385899b39 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -58,7 +58,9 @@ set(CMAKE_CXX_STANDARD 17)
 list(APPEND CMAKE_MODULE_PATH "${PROJECT_SOURCE_DIR}/cmake")
 
 # Valid values are "generic", "avx2", "avx512", "avx512_spr", "sve".
-option(FAISS_OPT_LEVEL "" "generic")
+if(NOT DEFINED FAISS_OPT_LEVEL)
+  set(FAISS_OPT_LEVEL "generic" CACHE STRING "Optimization level (generic, avx2, avx512, etc.)")
+endif()
 option(FAISS_ENABLE_GPU "Enable support for GPU indexes." ON)
 option(FAISS_GPU_STATIC "Link GPU libraries statically." OFF)
 option(FAISS_ENABLE_CUVS "Enable cuVS for GPU indexes." OFF)