O'Neill is a well-intentioned man who only want to help his students grow and learn, Unfortunately, he is also clueless, spineless, and more easily manipulated than a well-lubricated Rubik's Cube.

O'Neill seems to have a hard time remembering the names of any of his students, but by the same token doesn't forget one once he's learned it. He recognizes Trent on one occasion, even though it had been years and Trent didn't recognize O'Neill at first. This is particularly remarkable when one realizes that Trent probably wasn't much of a contributing student.

For some reason, O'Neill was in a fraternity in college. He was also a fan of James Dean. O'Neill tried out for the gymnastics team in high school, and managed to hurt himself fairly badly.

He has a steamy relationship with Ms. Barch that began at a field trip to a paintball range. Barch is definitely the more dominant one of the pair. They frequently pair off for wild sex in direct violation of school policy - it is unclear why Ms. Li does nothing about it. Every once in a while he and Ms. Barch argue, and O'Neill's response is usually to cry.

On one occasion, Ms. Barch became convinced that O'Neill had proposed marriage (when he actually did no such thing). With DeMartino's help, O'Neill was eventually able to summon the necessary backbone to set the record straight, and though Ms. Barch was initially enraged, she later decided she liked him with a little more guts.

O'Neill has a tendency to try "exciting new assignments" on his students. He often takes his inspiration from comments made by Daria or Jane, although on one occasion he took direction from a motivational speaker at a seminar and gave his students an assignment to deliberately fail at something - with predictably disastrous results. As part of his duties as Language Arts teacher, O'Neill is also the Drama coach and handles direction of school plays. He also fancies himself Daria's mentor in writing, though the reality is that she probably knows far more about literature and writing than he does.

During the summer before Daria's senior year, O'Neill ran a day camp for children called "The OK to Cry Corral" with the help of DeMartino and (reluctantly) Daria. O'Neill proved less then adept at dealing with children, as his feel-good prattling was generally ignored and the campers quickly grew tired of being cooped up all the time. In particular, O'Neill made a poor impression on Link, who accused him of not really caring - or at least sucking at it. By the end, O'Neill realizes his camp is a complete failure, except that it reawakens DeMartino's desire to teach.

He seems to be something of an expert on the life and times of Karl Marx, and may in fact be a communist sympathizer.

O'Neill suffers from seasonal allergies and carries an inhaler for when things get particularly rough. During a field trip to some nearby mountains, he nearly died.

He reads aloud from a motivational book each night and sleeps with recordings of nature sounds. His apartment is decked out with all the New-Age paraphernalia - candles, macramé, bonsai trees, wind chimes, you name it. He drinks St. John's Wort tea when depressed.

Helen feels that O’Neill has a screw loose. She's probably right.