Cyril O'Reily is a brother of Ryan O'Reily.
Cyril was a hoodlum like his brother (who's actually his half-brother, as was revealed by Suzanne Fitzgerald), Ryan, Cyril was Ryan's lieutenant and bodyguard in the Bridge Street Gang until he was injured in a fight, suffering brain damage. In the second season of Oz, Cyril murdered the husband of Dr. Gloria Nathan on orders from his brother, who was in love with Dr Nathan. Arriving in prison, Cyril was originally housed with Vernon Schillinger, head of the Aryan Brotherhood. After Schillinger raped Cyril, Ryan got his brother transferred to his own cell.
In the last season of the show, Cyril was executed despite Ryan's multiple attempts to save his brother, and despite Cyril's low mental capacity. Although Cyril is normally placid, he is prone to violent attacks on people when enraged, often showing great strength, which has earned him the resentment, but also the respect, of many inmates.