An expression meaning that one has been surprised by what has just occured. It is similar to saying "wow" or "holy crap" but implies a more laid back manner.
"So did you see the new... OH SNAP! A guy just ran buy with a katana!"
Used in taunting your manager when you make comments on his wife leaving him due to infidelity.
Me: "Hey scott I did your wife last night, not that it matters because she left your sorry ass anyway when she caught you screwing 18 year olds"
Derek: "OH SNAP!"
(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.