In On My Block season 4 episode 5, the fan-favorite character Oscar (Julio Macias) a.k.a. “Spooky,” the former leader of the Santos gang, is shot by an unknown attacker and killed right before he’s about to move out of Freeridge to start a new life— who exactly is responsible for Oscar’s death? Through a series of flashbacks in season 4, it’s revealed that Oscar amicably left the Santos gang two years prior and Cesar (Diego Tinoco) became the Santos' leader, taking Oscar’s place. Since Oscar didn’t appear to have any ongoing conflicts with the local gangs at the time of his death, it’s likely he was killed because of his affiliation with Cesar.
In On My Block season 1, Oscar is initially one of the Core Four’s main obstacles after he initiates Cesar into Santos. In seasons 2 and 3, Oscar becomes a good friend to the Core Four when he helps Ruby overcome his depression, comforts Monse after her mother’s suicide, and saves them from being killed by the Prophet$ by framing the rival gang for the Freeridge Savings robbery. In season 4, Oscar has a new family and plans to move to Portland to fulfill his dream job of owning a restaurant. Uneasy about leaving Cesar behind, Oscar finally convinces his brother to come with him to Portland when Oscar is shot and killed outside of his Freeridge home. While the series never reveals exactly who killed Oscar, On My Block insinuates that Oscar was likely killed by a rival gang member or enemy of Cesar.
Given the recent unease amongst the local gangs, there are many possibilities regarding the identity of Oscar’s killer. Before Oscar is murdered, news about the death of the Santos' boss Cuchillos begins to spread, which throws the hierarchy of the gangs into chaos. After Sad Eyes tells Joker that Cesar knew about Cuchillos' death and didn’t tell Santos, Cesar’s territories are given to Joker and he’s demoted within Santos. Cuete from the 19th Street gang breaks their truce with Santos and steals Cesar’s stash of cash he was saving for Monse, Ruby, and Jamal to replace the missing RollerWorld money. According to Officer Hammel, the rival gang the Prophet$ had also been “trickling out of jail.” Since it wasn’t yet common knowledge that Cesar was demoted within Santos, a member of the Prophet$ or 19th Street could’ve killed Oscar to challenge Cesar as Santos' leader.
On My Block appears to intentionally leave the identity of Oscar’s killer unanswered to focus more on the repercussions of his death. When Cesar looks for a place to spread Oscar’s ashes in episode 7, Cesar discards his plan of revenge and finds multiple ways to honor his brother’s legacy instead. While Cesar initially wants to gain revenge against Oscar’s killer, his discussion with Ricky Galindo in which he points out that revenge would hurt Cesar more than Oscar’s killer makes him reconsider. Cesar confronts Santos and officially leaves the gang after he realizes they didn’t help Oscar when he needed their protection the most. Cesar also decides to leave Freeridge to fulfill his own dream of becoming an architect to build the life Oscar deserved for himself. Oscar’s death therefore changes the trajectory of Cesar’s future and the series by having Cesar leave Freeridge, breaking the cycle of the Diaz family being resigned to the life of a Santos.
The viewer likely isn’t shown the identity of Oscar’s killer in On My Block because their identity is inconsequential. Before Oscar dies, Core Four member Cesar says “Who did this to you? Tell me!” Oscar smiles at his brother and shakes his head. By withholding the identity of his shooter, Oscar is ensuring that Cesar won’t be tempted to seek revenge, which could jeopardize the bright future he hopes Cesar will have and land him back in the Santos gang. By keeping the viewer in the dark along with Cesar, it suggests that the identity of the individual responsible isn’t relevant to the plot because Cesar and the rest of the Core Four's future doesn’t involve the Santos' politics and lies outside of Freeridge.