William Joseph Patrick O'Brien (November 11, 1899 – October 15, 1983) 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 and close friend James Cagney. O'Brien also appeared on stage and television.

O'Brien was born in 1899 to an Irish-American Catholic family in Milwaukee, Wisconsin. All four of his grandparents had come from Ireland. The O'Briens were originally from County Cork. His grandfather, Patrick O'Brien, for whom he was named, was an architect who was killed while trying to break up a saloon fight in New York City. His mother's parents, the McGoverns, emigrated from County Galway in the west of Ireland in the mid- to late-19th century.

As a child, O'Brien served as an altar boy at Gesu Church, while growing up near 13th and Kilbourn Streets in Milwaukee. He attended Marquette Academy with fellow actor Spencer Tracy, who was a lifelong friend. During World War I, O'Brien and Tracy joined the United States Navy. They both attended boot camp at the Great Lakes Naval Training Center, but they never went to sea. The war ended before their training had finished.

Jack Benny was also at the Great Lakes Naval Training Center at the same time as O'Brien and Tracy. According to his autobiography, Benny performed a number on the violin at a show one evening, when the sailors started booing and heckling him. O'Brien walked on stage and whispered in his ear, "For heaven's sake, Ben, put down the damn fiddle and talk to 'em." Benny stopped playing his violin and made a series of comments that got laughs from the audience. In this way, O'Brien indirectly helped to start Benny's career in comedy.

After the war, O'Brien finished his secondary schooling at Marquette Academy and later attended Marquette University. While still at college, he decided to seek work as an actor. He and Spencer Tracy moved to New York, where they both attended the American Academy of Dramatic Arts. The two struggling young actors shared a small studio apartment, and began their careers on the stage.

O'Brien spent a decade in plays on Broadway and in the New York City area.

O'Brien made his film debut in the Vitaphone Varieties short film, The Nightingale, produced in New York and released in August 1930. His first starring role was as ace reporter Hildy Johnson in the original 1931 version of The Front Page with Adolphe Menjou. In 2010, this film was selected by the National Film Preservation Board for preservation in the Library of Congress's National Film Registry as being "culturally, historically, or aesthetically significant."

He was the lead in Paramount's Personal Maid (1931), and appeared in a Metro-Goldwyn-Mayer musical Flying High (1931), supporting Bert Lahr. He was Irene Dunne's love interest in RKO's Consolation Marriage (1932), then co-starred opposite a young Bette Davis in Hell's House (1932). O'Brien stayed in leads, going from studio to studio: Scandal for Sale (1932), at Universal; The Strange Case of Clara Deane (1932), at Paramount; Hollywood Speaks (1932), American Madness (1932) and Virtue (1932).