Usually stated when realizing you are completely fucked and are about to be killed or be mortally wounded. Also used when someone has fucked up and is about to blow themselves or something important to kingdom come.
Guy: *plants a bomb on the building* Wait...that's OUR building! OH SHI- *Insert generic massive explosion here*
You say this when you are about to say Oh Sh!t ,but your parents walk in the room.
Person 1: *Stubs toe* Oh shi-
Parents: Walk in the room
Person: I mean God NO
Oh SHi- I mean God no is what you do when your about to say a OH SHIT then your parents or innocent kid walks into the room.