An OHSHIT moment is the instant between the pain and screaming, or the first time you smell Paris Hilton's cooch. (Maybe a mix of both.) Any sort of moment where you are bound to freak out has an OHSHIT, big or small. Here you may yell out a shitplitive, begin to cry, or crap yourself. Or all three.
(An example of an OHSHIT moment)
In Johnny's head-
'OHSHIT, definitely not having kids now.'