A person who has smoked too much opium and disintegrated their brain to the point of being retarded.

Someone people turn to when they're sad who makes them feel better.

A person who is highly addicted to Oxycontin aka O.C.s.
O- Heads usually crush up the pills and snort them, even possibly injecting them. If you deprive an O- Head of their "daily dose", you are in for three kinds of hell!