Little is known about Oscar's life before the apocalypse. Supposedly he was married and had a family, but became separated from them after he was arrested for breaking and entering.
While Oscar was in prison, the apocalypse took place. He had no idea as to the true nature of the events unfolding, only that mass panic and rioting were occurring within the prison. Oscar, along with fellow prisoners Axel, Andrew, Tomas and Big Tiny, was locked inside a cafeteria by a guard who went to find assistance but never came back. The group survived off of the food in the cafeteria kitchen for ten months, having no other human contact until Rick Grimes and his group of survivors entered the prison. Rick released the prisoners from the cafeteria and, after informing them of what had become of the world, struck a deal with them. He would help the prisoners clear out the walkers from one of the cell blocks and that would become their own separate living space, and Rick's group would take half of the remaining food in exchange. The prisoners were not entirely comfortable with this arrangement and Tomas and Andrew attempted to kill Rick and his party in order to keep the prison for themselves. This attempt failed and Tomas paid with his life as Rick slashed him with a machete and threw Andrew out and left him to the walkers. Oscar and Axel became the only two prisoners left and they swore to Rick that they weren't violent like the others, but Rick didn't trust them and demanded that they stay away from his people.
The following day, however, a large mob of walkers entered the prison. As it turned out, Andrew was still alive and had drawn the undead into the prison as well as set off the riot alarms to attract more of them. Rick, Daryl, Axel and Oscar attempted to shut the alarm off and were confronted by Andrew who attempted to convince Oscar to shoot Rick so that they could take the prison back for themselves. Instead, Oscar turned Rick's gun on Andrew and shot him dead. After this, Oscar and Axel were uneasily welcomed into the group and allowed to contribute to their survival.
In the episode "Made To Suffer", Oscar accompanies Rick, Daryl and Michonne as they set out to find the town of Woodbury after Glenn and Maggie are captured and taken prisoner there. They use their skills to discover and free the captives, but in the process, however, are immersed in an intense gunfight with the soldiers of Woodbury. Oscar assists the injured Glenn over the wall to escape, however, this exposes him and he is ultimately gunned down by one of the soldiers.