Short for "only child syndrome." Can be associated with selfishness, rudeness, laziness, and/or arrogance, but is not limited to these things. OCS may also be accompanied by strange/unexplainable side effects, i.e. excessive computer-nerdiness, knowledge of the names/charicteristics of people who do not know you, the need to bite inatimate objects or others, the need to snuggle with those you have just fought with, the urge to tickle/poke others for no apparent reason, and many other habits that vary by individual.
Person One: How does Angie know so much about hooking up her new computer?
Person Two: She's got OCS.
Person One: What are you doing?
Only Child: -bites Nintendo DS-
Person Two: Oh, don't mind her. She's got a bad case of OCS.