Del Mar racetrack
O'Neill was born in Dearborn, Michigan and moved to Santa Monica, California when he was 10, where his father, Patrick, took him to watch horse racing at Santa Anita Park. O'Neill became a hot walker while in high school, then went to work at Del Mar racetrack, and obtained his trainer's license in 1989. His brother, Dennis, is a bloodstock agent and helps select horses at auction for clients.