O'Leary auditioned for the lead in Home Alone 3 and made his acting debut in the lead role in 2000 in the made-for-television film Mom's Got a Date with a Vampire. He was subsequently cast in the thriller Domestic Disturbance, playing the son of John Travolta's character.
O'Leary next appeared in another thriller, Frailty, directed by Bill Paxton, and in the kids comedy Spy Kids 2: The Island of Lost Dreams, both of which opened in 2002 to positive reviews, gaining O'Leary recognition among teenage audiences. In 2003, O'Leary had a minor role in the third Spy Kids film, Spy Kids 3-D: Game Over, and also appeared in the 2004 drama The Alamo, although most of his role was reduced to one line.
In 2005, O'Leary had a role in Warm Springs, a television film, and Havoc, a drama starring Anne Hathaway that was released directly to video. O'Leary rose to mainstream prominence in the latter half of the 2000s for his roles in Brick, a thriller starring Joseph Gordon-Levitt, and the 2007 films Live Free or Die Hard and Death Sentence. He appeared as Garret in the 2009 film Sorority Row and as Johnny Koffin in Mother's Day, released the following year.
In 2011, O'Leary won the Breakthrough Performance award at the 2011 SXSW Film Festival for his work in the film Natural Selection, with Rachael Harris.
