gcc has a -Og option that performs all optimizations that don't interfere with debugging, so I'd guess that emacs..-Og is built using gcc's -Og option and emacs

gcc has a -Og option that performs all optimizations that don't interfere with debugging, so I'd guess that emacs..-Og is built using gcc's -Og option and emacs..-O2 is built using gcc with the -O2 option.