OMG (Also 'Oh Crap') Face is a rage comic character often used in the end of various strips to express really extreme emotions. The face is highly detailed with lots of shading compared to the usual Rage Faces, and it even has a dark background behind it. It is known for being more realistic-ish and detailed than most of the other faces, so it's often shown to show extreme emotion.
It also has a few counterparts or something. The most popular ones are the red-eyed counterpart and the wide-mouthed counterpart.