diff --git a/Modules/main.c b/Modules/main.c
index b4bece6cabe..757aa77afb4 100644
--- a/Modules/main.c
+++ b/Modules/main.c
@@ -68,7 +68,7 @@ Options and arguments (and corresponding environment variables):\n\
 -d     : debug output from parser (also PYTHONDEBUG=x)\n\
 -i     : inspect interactively after running script, (also PYTHONINSPECT=x)\n\
          and force prompts, even if stdin does not appear to be a terminal\n\
--O     : optimize generated bytecode (a tad)\n\
+-O     : optimize generated bytecode (a tad; also PYTHONOPTIMIZE=x)\n\
 -S     : don't imply 'import site' on initialization\n\
 -t     : issue warnings about inconsistent tab usage (-tt: issue errors)\n\
 ";