Pat O'Brien (November 11, 1899–October 15, 1983) was an American film actor with more than one hundred screen credits.
O'Brien was born William Joseph Patrick O'Brien to an Irish-American Catholic family in Milwaukee, Wisconsin. He served as an altar boy at Gesu Church while growing up near 13th and Clybourn streets. O'Brien attended Marquette Academy with fellow actor Spencer Tracy, and later attended Marquette University. Reportedly he also served with Jack Benny at Great Lakes Naval Station during World War I.
O'Brien appeared with James Cagney in nine feature films, including Angels with Dirty Faces (1938) and Cagney's last film, Ragtime (1981). He began appearing in movies (many times playing Irish cops or priests) in the 1930s, starting with the role of ace reporter Hildy Johnson in the original version of The Front Page in 1931. He appeared in the highly successful 1946 suspense film, Crack-Up and played the lead in The Personality Kid.