1) used to describe a dangerous person, one who is so violent, he is simply named "Oh fuck"
2) the last thing uttered before something unfortunate happens to you, particularly to your bottom.
To express immense surprise/pain, like when you light that firework strapped to your dick or when you circumcise yourself by tying a rope to your pp and looping the other end around a trailer hitch attached to a ford and telling the driver to “PUNCH IT BITCH”