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Oscar-129 was conscripted into the SPARTAN-II program in 2517, at age six, when Dr. Catherine Halsey singled him out as a physically and intellectually superior child. He was then replaced with a Flash Clone and taken to Reach, where he was trained by Deja and Chief Petty Officer Franklin Mendez alongside the rest of the Spartan children.
A few weeks after the augmentation procedures, Oscar, along with Daisy-023, Ralph-103, Joseph-122, and another Spartan, rebelled against their trainers, holding them at gunpoint in front of Dr. Halsey in order to convince her to let them leave. Halsey granted them freedom, and they escaped the training facility, hoping to return home to their parents.
Later he would return to his home in New Houston to discover his flash clone in his place. His parents would call the police to report a break in. Feeling like there was nowhere to go, he wandered over to Wickers Park and shot himself.