Extremely fond of the number "0", as it is the only grade he enjoys giving.
Also known to creep on certain innocent girls through entire class periods/school years
Student B- Oh, dude, that blows. Enjoy failure...
O'Brien- So you put this here, and don't skip steps, OR ITS A ZERO! Oh wait, I did it wrong...and I skipped a step...ITS STILL A ZERO FOR YOU!
Student 1-Wait, I don't understand this, could you go over it?
O'Brien-NO! Quiz time!
We're not sure how he got out of this dilemma, as the show ended with him still in grave danger, but he seemed okay in the next episode.