OP is an abbreviation for Original Poster, AKA the asshole who got there first. Used most commonly on *chan boards and forums, the quality of a thread usually balances upon the OP's ability to generate drama and/or interest in whatever bullshit he's yelling into cybarspace, although occasionally Anonymous just grabs something by the scrotum and runs with it. A thread that snowballs into win can often give an OP unwarranted self-importance, leading to faggotry and/or lulz if they attempt to take credit for the hard work of legion.