"Oh bust" is an exclamation, much like the late "you got dissed." It signifies that someone has been taken down a knotch. When someone utters "oh bust" they are to cuff their right hand over their mouth as if to holler. After "oh bust" is said a laugh should promptly follow.
Jim: Hey Steve didja go to that party last night?
Steve: What party? There was a party? How come I wasn't invited to the party?
Stacy: You have no friends.
Jim: Oh bust! Hahaha