In some parts of the U.S. "Oh, Snap!" is an exclamatory remark. It is very similar to "Oh, No!" or "Oh My Goodness!" Sean is right in that it is sometimes meant to indicate that someone or something has "crossed the line of common decency;" but sometimes it is used to indicate a "superlative" or something well done. For example, if a diver makes an extraordinary dive from a high diving board, someone who thinks that that dive was extraordinarily good might say: "Oh, Snap!" But on the other hand, if that dive was extraordinarily bad and the diver was hurt, someone watching it might say "Oh, Snap!!!" It can vary from situation to situation and from user to user. We Americans are not very consistent.

"Oh Snap”　は「このまま不味い~」のようです。 購読　－＞「ゲッ」

My boyfriend just asked about this phrase to me the other day, actually. I think everyone has given good answers...it means so many different things!

Australians don't use this expression.

I agree with all three answers. I do not think this phrase is outdated, though. I use it all the time! Oh snap! I have to do my homework! (Literally...) :)