Phrase used to describe an "obsessive compulsive" trait. If the person seems to have an repetitive desire, or somewhat recurring and absurd need, then he/she will be regarded as "OC"
"I need to type up all of my history notes from today." -"Dude, you're so OC"
"Black pens are all I write with; color disturbs me"
"Dawg, you're OC"