Oscar is first seen at home of a drug dealer Freebo (who is Dexter's selected victim), attacking Freebo with a knife. Dexter's sudden appearance distracts Oscar long enough for Freebo to escape and Oscar immediately turns his attention to Dexter. The two struggle and Oscar knocks a loose crown from Dexter's teeth, before Dexter manages to turn the knife around and stab him in self-defense. A shocked Oscar gasps "Who are you?" as he falls back against a wall, to which Dexter replies "Who are you?" Seconds later Oscar was dead (which traumitizes Dexter for killing an innocent).
Oscar's body is seen the next day at the crime scene, where his brothers Ramon and Miguel Prado are also present, as well as at his funeral which Dexter attends, because Miguel had invited him, and Dexter wants to learn more about the man he killed, concerned that he broke "the code."
At first it is believed that Oscar died doing his duty as a community organizer who was simply confronting the local drug dealer, but Debra manages to learn from a police informant Anton that Oscar was actually a drug user and in Freebo's debt. Though he was not a killer, Oscar's drug use and attempted murder of both Freebo and Dexter made Dexter feel better about having killed him.