an oh-snap is a way of saying g-string, with out any of the guys knowing what you are saying. for instance, if you want to discuss what color every ones is, use the term oh-snap.
girl one: omg....u can TOTALLY see her oh-snap. what a slut.
girl two: HEY! IM WEARING AN OH-SNAP TOO!
girl one: oooh. 0.0 oh crap!