diff --git a/pkgs/by-name/op/openmolcas/package.nix b/pkgs/by-name/op/openmolcas/package.nix
index c3b43cf20f3e..3630ed9622b6 100644
--- a/pkgs/by-name/op/openmolcas/package.nix
+++ b/pkgs/by-name/op/openmolcas/package.nix
@@ -117,6 +117,9 @@ stdenv.mkDerivation rec {
 
   passthru = lib.optionalAttrs enableMpi { inherit mpi; };
 
+  # fix build with GCC 15
+  env.NIX_CFLAGS_COMPILE = "-std=gnu17";
+
   cmakeFlags = [
     "-DOPENMP=ON"
     "-DTOOLS=ON"