1997

Cyril and his brother, Ryan O'Reilly, were in a fight, and Cyril got hit hard on the head, giving him the mind of a five year-old.

1998

Ryan had Cyril kill Doctor Gloria Nathan's husband, and Cyril was caught and sent to Unit B, where he became the new girlfriend of Vern Schillinger. Ryan got Cyril transferred to Emerald City so he could protect him.

1999

Ryan got Cyril entered into a boxing championship. Cyril beat Jim Robson, then Chucky Pancamo, and got into the championship match against Hamid Kahn. Cyril beat Kahn so badly, Kahn became braindead. Out of anger, Cyril beat up Ryan.

2000

Cyril's guilt from causing Kahn's death caused Ryan to sign them both up with meeting Nathan and creating closure. This backfired, and Cyril felt even more guilty. When Mondo Browne tried to rape Cyril, Cyril freaked out and had to be put on medication, which he eventually overdosed on. He spoke to Nathan about Ryan, and was eventually returned to Emerald City. When he found out about Ryan's affair with Officer Claire Howell, he threatened to tell.

2001

Reporters from television (and Ryan and Cyril's past) arrived in Oz, and Cyril eventually beat one up. His medication was switched to keep him calm. He and Ryan signed up for an experimental medical program that aged inmates, and Cyril was the only one who began to age. When another inmate died, the testing ended and Cyril returned to normal. However, inmate Jia Kenmin picked a fight with Ryan, and Cyril knocked Jia into a coma. He was sent to Protective Custody for a while, but eventually returned to Ryan.

2002

Ryan's mother, Suzanne, arrived to work in Oz, and adopted Cyril as her own son. Jia returned, apprently redeemed, and forgave Cyril. When Ryan learned that inmate Li Chin wanted to rape Suzanne, Ryan attacked him with a shank. A fight followed, and Cyril stabbed Li to death. He was sentenced to death and sent to death row.

2003

He was photographed for a magazine's Death Row fashion spread. Governor Devlin found the Cyril case becoming a subject of controversy and a thorn in his side. He decided that, the only way to shut the people up is to make Cyril appear normal. He convinced a psychologist to suggest ECT (electro shock therapy) and got Cyril's father to sign a paper allowing this. Cyril began undergoing ECT, and afterwards was drooling and disoriented when Suzanne and Ryan came to see him. Cyril continued his ECT treatments, which was giving him slight memory loss. Ryan spoke to Cyril that he was going to have "one more special ECT treatment" and he'd be able to have anything he wants for dinner, and that Ryan would be able to be with him. Cyril was ecstatic, even though Ryan told him he'd have to lose his hair. At the barber shop, Cyril's head was shaved bald. Ryan spent the night with Cyril in his death row cell. Suzanne came to Cyril and taught him a special prayer before she and Ryan were kicked out of death row. Cyril was given his favorite of Ryan's shirts to wear. As Cyril was being led to the chair, the inmates of Emerald City pounded on the glass of their cells in respect of him. Cyril was strapped to the electric chair, and when Glynn told him he can make any last statement he likes, Cyril chose: "Huh?" Sister Pete held Suzanne's hand as the men began to charge the chair. Glynn's phone rang, and after he hung up he announced that the state supreme court has issued a Stay of Execution for Cyril O'Reilly for the immediate future. Cyril was told that his father, Seamus, had come to Oz. The day of Cyril's execution arrived, and he was told once again that his "special ECT treatment" had arrived. He was led into the room and actually electrocuted this time.