He made his debut in the made-for-television Disney Channel Original film Mom's Got a Date with a Vampire (2000), and would go on to star in the thriller Domestic Disturbance (2001) opposite John Travolta.

In 2011, he starred opposite Rachael Harris in the critically acclaimed independent film Natural Selection, followed by a lead role in Fat Kid Rules the World (2012).

He had a minor part in Gore Verbinski's The Lone Ranger (2013), and lead roles in the horror films Stung (2015) and Bokeh (2017).

[1] 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.