A person who alternates between Mr. Right and the Douchebag Next Door. You can recognize him by refusal to communicate while in a relationship and by a wrecked set of priorities. This guy will become a permanent asshole a month before breaking up with you.
The epitome of a S.O. would be Sean Oliver at UPenn, but S.O.s can be found throughout the country. Most abundantly at college campuses and local bars.