"O ma ga" is an awesome variation on oh my god. This term can be used in dire situations, sarcastic situations, and exciting situations, although its use is not limited to those times. It is also fun to say it randomly.
Person 1: I just got a 98 on my final! :D
Person 2: O MA GA!!!
Person 1: I think I lost my cell phone. D:
Person 2: O MA GA!!
Person 1: I think you should stop saying "o ma ga" so much. -.o
Person 2: O MA GA... no.