(exclamatory phrase) a playful indication of surprise, misfortune, or insult
popularized by Tracy Morgan of Saturday Night Live, OH SNAP is seemingly derivative of oh no you didn't
where an insulted person, for example a guest of Jerry Springer or often a spunky African American woman contends the insult being made against him/her. While derivative of "oh no you di-int," OH SNAP has more of an emphasis on playfulness and can be said by people other than those being insulted.
see also: OH ZIP
Ted: Man, where you been all afternoon?
Darrin: I had to stop off at your mom's place for a nooner
that last a bit longer than expected.
Ben: OH SNAP!