Cyril O'Reily is a fictional character, played by American actor Scott William Winters, on the HBO drama Oz.
Cyril was Ryan's lieutenant and bodyguard in the Bridge Street Gang until he was injured in a fight, suffering brain damage.
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.
Later, upon considering what Cyril's life in prison would be like without his brother to protect him, Ryan finally confesses to orchestrating the murder of Preston.
Ryan, along with Cyril, Chris Keller and Tobias Beecher, who all have a vendetta against Schillinger, devise a plan to turn father against son.
Khan is hooked up to life support which creates animosity among the black inmates and ratchets the racial tension up to an all-time high.
Ryan suggests psychotherapy with prison psychologist "Sister Pete" Reimondo, who thinks interaction sessions with the families of Cyril's murder victims is the best way to clear his conscience.
In Em City, Cyril is later attacked by inmates Leroy Tidd and Mondo Browne, who threaten to rape him.
With the help of Sister Pete, Ryan convinces Dr. Nathan that he wasn't responsible, and as a result she resumes Cyril's treatment.
In the midst of this, Em City is undergoing a demographic shift that worries Ryan, Chris Keller and some other white inmates.
The new unit manager, Martin Querns, an African American, has empowered the Homeboys to do whatever they wish in exchange for enforcing a state of non-violence in Em City.
Claire Howell, one of the few white officers left, then develops a sexual relationship with Ryan which is discovered by Cyril.
With the help of Muslim leader Kareem Said, they obtain videotaped evidence showing the prohibited behavior that the Homeboys are being allowed to commit, forcing Warden Glynn to fire Querns.
After Asian inmate Jia Kenmin arrives in Oz, disputes erupt between him and both the Irish and Latino factions.
Back in Em City, Ryan and new inmate Padraig Connolly, an IRA member awaiting deportation, plan to blow up Oz.
Governor Devlin however reduces her charges to 2 years of community service at Oz where she can see Ryan and Cyril.
They enjoy seeing her on a daily basis but this is short lived when Jia Kenmin and new Asian inmate Li Chen attend some of the performing arts classes.
In an attempt to set up the O'Reily brothers, Kenmin pays inmate Glen Shupe to tell Ryan that Chen plans to rape Suzanne and as a result, he and Cyril attack Jia.
Cyril picks up a knife from the two Asians during the fight and stabs Li Chen to death defending Ryan.
In a meeting with other family members to raise money for a better lawyer, Ryan, their Aunt Brenda, Suzanne, and abusive father Seamus decide that Cyril is better off dying than staying the rest of his life in Oz or a mental institution.
A new inmate, Father Daniel Meehan, offers to help and riles up Sister Pete and some anti-death penalty activists to protest that sentencing a man in Cyril's mental state to death is cruel and unusual.
Meanwhile, Ryan exacts revenge by getting the Latinos to injure Glen Shupe and manipulating the COs to kill Jia Kenmin.
In effort to overturn Cyril's death sentence Ryan, with the help of Sister Pete and Father Daniel Meehan, gets activist attorney Arnold Zelman to appeal his case.
The appeals remonstrating the state's decision to kill a mentally handicapped person gather public support.
In light of this disturbing behavior, Jericho is taken away from Cyril, who begins sobbing uncontrollably over the loss of his hand puppet.
In the meantime, Governor Devlin, in an attempt to make Cyril appear normal, orders electroconvulsive therapy to be done to him.