- The Irish Mob: Subverted; they're not really an organized grouping, and many weren't part of any formal gang on the outside. The closest thing we get are Cyril and Ryan, who led their own Irish gang before coming to Oz.
- Mauve Shirt: Every member of the Irish whose surname isn't O'Reilly. Timmy Kirk is a subversion. As a member of the Irish, he is definitely a minor character, but when he joins The Christians, he becomes a significant character.
- Oire Land: Padraic Connolly, the rest of the Irish have more of a Southies style to them.
- The Troubles: Again, Padraic Connolly.
- White Gang-Bangers
Ryan O'Reily's mentally-handicapped younger brother, who suffered a head injury protecting Ryan and now requires constant supervision. An honest and good-hearted person, he is prone to outbursts of complete rage and is easy to fool — a bad combo in a place like Oz, which is full of Manipulative Bastard types.
- Beware the Nice Ones: He's endearingly innocent most of the time. But when he goes off, its best to be somewhere else.
- Butt-Monkey: Cyril gets it bad. Being Ryan O'Reily's brother and being rather soft in the head makes him a huge target.
- Crouching Moron, Hidden Badass: He's VERY good at boxing.
- Dumb Is Good: Before the brain damage, he was no better than Ryan in the morals department. Afterwards, he's good-natured to a fault.
- Dumb Muscle: Mentally handicapped and one of the strongest prisoners in Oz.
- Hair-Trigger Temper: Thanks to the brain damage, he has very poor impulse control. His response to distress is almost always to beat the hell out of the offender.
- I Want My Mommy!: Spends his final moments before getting the chair crying for his mom.
- Morality Pet: A weak one, for Ryan.
- Muscles Are Meaningless: Look at him, scrawny, yet one of the best fighters in Oz.
- Psychopathic Manchild: He's got the brain of a 5-year-old and the temper of a homicidal maniac.
- Sanity Slippage: The entire 'Jericho' arc.
- Siblings in Crime: with Ryan, although he probably doesn't understand what's happening much of the time and is more of an Unwitting Pawn.
- Before the accident that caused his brain damage, Cyril ran in gangs right alongside Ryan.
- You Wouldn't Like Me When I'm Angry: You really, REALLY wouldn't.
Arrested and convicted for putting his baby in a rat-infested dumpster, he was part of Ryan O'Reily's Irish prison gang and colluded with O'Reily on several occasions regarding possible friction between both the Irish and the Homeboys, as well as in the aftermath of the prison riot. He for a while worked on janitor duty in Death Row, and delivered messages between Simon Adebisi and Shirley Bellinger.
- Ascended Extra: Pretty much existed as an O'Reily subordinate who saw Shirley Bellinger's vagina before getting a major role in Seasons 4 through 6.
- Church Militant: Hoo boy, is he a dark example. His temporary brush with religion turns him into a psychopath willing to kill anyone who apostatizes.
- Cruel and Unusual Death: Electrocuted to death when Hoyt jams a light fixture in his mouth.
- Depraved Bisexual: He'll offer a blowjob to anyone who'll listen, but he's also shown to be attracted to Shirley Bellinger.
- A Devil Am I: When he goes of the deep end, he becomes convinced that he is Satan himself.
- Evil Redhead: The evilest ginger in Oz.
- Face of an Angel, Mind of a Demon: He may look like a choir boy, but that's not even close to being accurate.
- Faux Affably Evil: He's pure evil with a smile.
- From Nobody to Nightmare: He starts off as an errand boy for Ryan O'Reily. He later becomes one of the most dangerous prisoners in Oz.
- Heel Faith Revolving Door First Catholic, then Protestant, then...Satanist. Then proclaiming to be Satan himself. He's nothing if not willing to change.
- In the Blood: Kirk's mother has a brief story arc. She... turns out not to be much more in touch with reality than her son.
- Knight Templar: He maims and murders for the lord. When Cloutier tries to get him to back off, he takes over his leadership of the Christians and seals the good reverend behind a wall.
- Manipulative Bastard: He loves to play innocent to get ahead. Rev. Cloutier and Father Mukada get it the worst, both temporarily losing their positions to his machinations.
- Sanity Slippage: He goes from not all there to psychopathic killer to declaring himself to be Satan in the flesh.
- Smug Snake: Without someone under his control, he's pretty pathetic.
- The Unfettered: Treats the Moral Event Horizon like a jump rope in his quest to destroy both Cloutier and Mukada.
- With Great Power Comes Great Insanity: When he gains control of the Christians, he loses his tenuous grasp on sanity.
- Would Hurt a Child: His own child, who he dropped in a dumpster to be eaten alive by rats.
The abusive, aggressive father of Ryan and Cyril. He's sent to OZ after being convicted of murder.
- Abusive Parent: He regularly and viciously beat Ryan and Cyril when they were growing up.
- The Alcoholic
- Butt-Monkey: His life has been one long bout of misery. He took it out on his wife and kids, which just made his life get worse.
- Jerkass: The man has zero redeeming qualities. Hell, he makes Ryan look downright friendly by comparison.
- Never My Fault: Seamus refuses entirely to accept responsibility for the horrible things he's done, remaining unrepentant and indignant.
- Your Cheating Heart: He cheated on his wife frequently, and even bragged about it to her face.
A soldier of the Irish and one of Ryan O'Reilly's henchmen. Replaced Timmy Kirk's position after Kirk joined The Christians.
- Cool Old Guy: He ends up being the closest thing to a father Ryan O'Reily ever had.
- Good Shepherd: He's very, very good at what he does. In fact, the only reason he's in Oz is that he was too good of a priest: he struck a police officer who was hassling one of his parishoners during a protest.
- Irish Priest: A textbook example.
- Nice Guy: He barely knows O'Reily and yet he makes a point of helping Cyril and attempting to renew Ryan's faith in the Catholic Church.
- No Dead Body Poops: Averted.
- Oire Land: Played by a native Irish actor, though never explicitly mentioned in the show.