diff --git a/Doc/using/cmdline.rst b/Doc/using/cmdline.rst
--- a/Doc/using/cmdline.rst
+++ b/Doc/using/cmdline.rst
@@ -210,14 +210,15 @@
 
 .. cmdoption:: -O
 
-   Turn on basic optimizations.  This changes the filename extension for
-   compiled (:term:`bytecode`) files from ``.pyc`` to ``.pyo``.  See also
-   :envvar:`PYTHONOPTIMIZE`.
+   Remove assert statements and any code conditional on the value of
+   :const:`__debug__`.  Change the filename extension for compiled
+   (:term:`bytecode`) files from ``.pyc`` to ``.pyo``.
+   See also :envvar:`PYTHONOPTIMIZE`.
 
 
 .. cmdoption:: -OO
 
-   Discard docstrings in addition to the :option:`-O` optimizations.
+   Discard docstrings in addition to what :option:`-O` does.
 
 
 .. cmdoption:: -q
diff --git a/Modules/main.c b/Modules/main.c
--- a/Modules/main.c
+++ b/Modules/main.c
@@ -66,8 +66,9 @@
 -i     : inspect interactively after running script; forces a prompt even\n\
          if stdin does not appear to be a terminal; also PYTHONINSPECT=x\n\
 -m mod : run library module as a script (terminates option list)\n\
--O     : optimize generated bytecode slightly; also PYTHONOPTIMIZE=x\n\
--OO    : remove doc-strings in addition to the -O optimizations\n\
+-O     : remove assert and __debug__-dependent statements; change .py\n\
+         to .pyo; also PYTHONOPTIMIZE=x\n\
+-OO    : remove doc-strings in addition to the -O changes\n\
 -q     : don't print version and copyright messages on interactive startup\n\
 -s     : don't add user site directory to sys.path; also PYTHONNOUSERSITE\n\
 -S     : don't imply 'import site' on initialization\n\