O'Donnell served as an officer in the Atlanta Police Department, and was a close friend and fellow academy rookie to Dawn Lerner. He was married to a wife, and together they had a child who was born in Grady Memorial. During the outbreak he was one of the officers tasked with evacuating the very same hospital, and then staying behind to help sweep the building for remaining civilians. He and his team then witnessed the bombing of Atlanta, and he survived up until the show's events, though not without losing much of his humanity. The fate of his family is unknown but dialogue suggests they were killed during the outbreak as he never mentioned any hope of reuniting with them, which could have been a contributing factor to the loss of his humanity.
Officer O'Donnell brings in an injured Gavin Trevitt to the hospital with Officer Amanda Shepherd. Later, he is seen walking with Lieutenant Lerner and they check up on Officer Gorman harassing Beth Greene. When Beth tries to flee, she is restrained by O'Donnell as Officers Shepherd and Alvarado dispatch of the zombies at the hospital entrance.
Officer O'Donnell and officer Alvarado are seen patrolling the streets of Atlanta, looking for Noah. When survivor Carol Peletier exits a building, she is suddenly hit by their vehicle. O'Donnell and Alvarado load Carol onto a gurney and put her into the car to take to the hospital.
Officer O'Donnell argues with Dawn Lerner over keeping Carol alive, stating that it is a waste of resources. After Beth intervenes, Dawn tells O'Donnell to alert Dr. Steven Edwards that Carol is to be taken off the machines.
O'Donnell berates Percy for not properly stitching a hole in his sleeve and shoves him to the ground. Beth looks on. He starts trying to draft her into service, but Dawn calls her away. Beth then walks in on them, catching them in the conversation. O'Donnell says it's time to make a change, that either Dawn can admit the situation to the rest of the officers or he will. Dawn pulls her gun on him and the two argue. He comes after her, knocking her gun down the elevator shaft and the two fight. He starts to get the better of her and Beth steps in to help. When he's distracted by Beth, Dawn is able to seize the opportunity to regain the advantage and therefore punches him in the throat before Beth shoves him down the elevator shaft, falling to his death. His body may have been eaten by the walkers at the bottom of the shaft.
He confronts Dawn about her leadership of the hospital and the fact that she covered for Beth who had been responsible for Gorman's death, the confrontation soon turns violent as he and Dawn end up in a fight during which O'Donnell attempts to strangle Dawn but gets distracted briefly when Beth tries to save her, this minor lapse in concentration gives Dawn the window she needs and as a result she strikes O'Donnell in the throat and kicks him towards the open elevator shaft before yelling at Beth to give the winded officer the final push to his demise. Since his brain was not destroyed to prevent reanimation, it can be inferred that he turned.
This list shows the victims Officer O'Donnell has killed:
- Numerous counts of zombies.
- "That guy is gone. We're supposed to protect people, to help them, but look at you. You're beating the old man. You're laughing with your buddies about that poor girl getting raped. That's who you are now."
- —Dawn to O'Donnell about his negative change in person[src]
Dawn and O'Donnell seemed to have a close relationship before the apocalypse started. He stated that they were rookies together and that they had shared a cigar when his wife gave birth to their daughter. Their relationship appeared to have deteriorated over time, with O'Donnell becoming violent and laughing at the female ward being raped. It worsened to the point where they were willing to kill each other, to which Dawn and Beth did kill him fearing for their lives.
- "What good is this place, if doesn't try to save people?"
- —Beth to O'Donnell about his intention to end Carol's medical treatment[src]
O'Donnell and Beth are shown to disapprove of each other during the short amount of time they spent together. He caught Beth and stopped her from escaping the hospital in "Slabtown". Later, the two argue over the fate of Carol Peletier as O'Donnell attempts to get Dawn to order Edwards to turn off the machines keeping her alive, angering Beth. In "Coda", O'Donnell suggests to Dawn that she should get rid off Beth, much to her annoyance, but Dawn refuses. Soon after, Dawn orders Beth to push O'Donnell down the elevator shaft after a brief fight between the two, to which Beth does not hesitate.
Gorman and O'Donnell were assumed to have a good friendship although they are not seen interacting at any point. Dawn points out that he is "Laughing with [his] buddies about that poor girl getting raped". Both Gorman and O'Donnell were one of the "bad ones" who raped some of the female wards, so it can be assumed that Gorman was the one O'Donnell was laughing with.