William Joseph Patrick O'Brien was an American film actor with more than 100 screen credits. Of Irish descent, he often played Irish and Irish-American characters and was referred to as "Hollywood's Irishman in Residence" in the press. One of the best-known screen actors of the 1930s and 1940s, he played priests, cops, military figures, pilots, and reporters. He is especially well-remembered for his roles in Knute Rockne, All American (1940), Angels with Dirty Faces (1938), and Some Like It Hot (1959). He was frequently paired onscreen with Hollywood star James Cagney. O'Brien also appeared on stage and television.

Knute Rockne, All American is a 1940 American biographical film that tells the story of Knute Rockne, Notre Dame's legendary football coach. It stars Pat O'Brien as Rockne and Ronald Reagan as player George Gipp, as well as Gale Page, Donald Crisp, Albert Bassermann, Owen Davis Jr., Nick Lukats, Kane Richmond, William Marshall and William Byrne. The film also includes cameos by legendary football coaches "Pop" Warner, Amos Alonzo Stagg, William H. Spaulding and Howard Jones, playing themselves.