Mark Anthony O'Neil was born on a far off, long forgotten outer colony world during the early Insurrection. He lived a normal life for most of his childhood, living in lower middle class family and doing fairly well in school.
O'Neil would graduate near the top of his class
"Lieutenant Commander, Mark O'Neil, UNSC Navy. Graduated three years early from the Luna OCS at the top of his class. Fought during conflicts on Arcadia, Miridem, and ultimately Reach. Was leading a covert operations squadron during the planet's fall, witnessed the massacre of the entire group. He's since been a traumatized drunk, living on a backwater colony world. It's quite a shame to see such a decorated officer end up like this."
―Lieutenant Rose Zhenya, Office of Naval Intelligence