O'Neill began his acting career on the stage, after dropping out of college to join a traveling theatre company. He served in the military in World War I, then returned to the stage. In the early 1930s he began appearing in films, including The Big Shakedown (1934) with Charles Farrell and Bette Davis, the Errol Flynn/Olivia de Havilland Western Santa Fe Trail (1940), the Frank Sinatra/Gene Kelly musical Anchors Aweigh (1945), The Green Years (1946), and The Reckless Moment (1949). His last film was The Wings of Eagles (1957), starring John Wayne.