He played Special Agent Ron Butterfield, the head of President Josiah Bartlet's Secret Service detail, on The West Wing,[5] and CTU Administrative Director Richard Walsh in the first two episodes of 24.

He starred in the Season 6 two-part finale of Grey's Anatomy as Gary Clark, the broken widower who holds the hospital hostage with a 9mm while pursuing the doctors responsible for unplugging his wife's life support.

[9] His most notable film performances occur in Seabiscuit, Secondhand Lions, Transformers, Dancer, Texas Pop.

81, Traffic, Sea of Love, A Quiet Little Marriage, Nothing but the Truth, Green Zone, J. Edgar (directed by Clint Eastwood), and Dallas Buyers Club.

[10] O'Neill splits his time between Los Angeles and Birmingham, Alabama,[1] where he lives with his wife, Mary, and three daughters, Ella, Annie, and Molly.