[MtL]

There is no difference at all, since the latter takes precedence (read the man pages):

$ gcc -c -Q -Os -O3 --help=optimizers > Os-O3-opts

$ gcc -c -Q -O3 --help=optimizers > O3-opts

$ diff Os-O3-opts O3-opts

$

[rlpb]

I didn't realise that. He actually uses -Os after -O3.