A variant of "oh shit", "oh snap", or "oh dip". More truely a variant of oh shit, used in places where saying shit would be unacceptable, I.E. a classroom or public place. Is often said with a playful gangsta tone, being pronounced "Oh siht" having the T sound cut short.
Teacher: Tommy, you got a 64 on the exam.
Random Classmate: Oh sit son.