http://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html says about -O:
-O also turns on -fomit-frame-pointer on machines where doing so does not interfere with debugging.
and about -fomit-frame-pointer:
Enabled at levels -O, -O2, -O3, -Os.
So, it is not clear whether -fomit-frame-pointer is always enabled at level -O or not.
This bug is related to bug 51019 (unclear documentation on -fomit-frame-pointer default for -Os and different platforms).