Oscar Guirerro is the main antagonist of the Fillmore! episode "Next Stop, Armageddon". He is X Middle School's best model railroad engineer until his dismissal.
He was voiced by Kyle Sullivan.
Oscar first appeared attending the Model Train Convention where Fillmore and Third are assigned conventional duty. But things went wrong when a series of mishaps caused many train sets to be broken down through chain reactions, ruining the entire convention.
While investigating the wreckage of the train sets, Third found a Speedy-O Mite remote control that caused one of the trains to fall off the track that triggered the crash. Fillmore and Third traced the remote control back to a student named Ralph Eddie of the RC Car club, but something else turns out: according to security footage, there was a Speedy-O-Mite car in Oscar's train set that triggered the crash in the first place.
Fillmore and Third soon learned that Oscar's father owns a model train shop and that RC cars were being popular at that time, which makes Oscar's family on the verge of going out of business and moving out of town. This made Fillmore and Third realize that Oscar engineered the crash and tried to frame the RC car club so that he can sell his family's train sets and supplies and that his family will not either lose their business or have to move.
With that in mind, Fillmore and Third confronted Oscar for setting up the crash with the evidence. Oscar tried to escape, but ends up being caught by the duo. Oscar explained that he didn't want his family to lose everything, but Fillmore stated that he has to move on despite his losses. It was implied that Oscar is taken into custody and kicked out of the train club for his villainous actions.