Graduated from Auburn University.
O'Neill grew up in Montgomery, Alabama. In 2011, after many decades living outside of the state, he bought a house in the Cahaba Heights neighborhood of Birmingham, Alabama, so that he could spend more time with his ailing father. Even after his father's death, he still splits his time between Birmingham and Marina Del Ray, California.
He has three daughters: Ella, Annie, and Molly O'Neill.
Majored in Economics at Auburn University.
As of 2016, has appeared in three films that were nominated for the Best Picture Oscar: Traffic (2000), Seabiscuit (2003) and Dallas Buyers Club (2013).