U.S. Democratic Party political organizer, government official, and sports executive.

O'Brien received a bachelor of law degree from Northeastern University, Boston (1942). A brilliant political strategist, he managed a victorious (1948) congressional campaign for his boyhood friend Foster Furcolo and then John F. Kennedy's (Kennedy, John F.) 1952 and 1958 U.S. Senate victories. After managing Kennedy's narrow 1960 presidential victory he was named (1961) special assistant to the president for congressional relations and continued in that position after Kennedy's assassination. He was instrumental in winning approval for legislation introduced by the White House, including the bills creating the Peace Corps and Medicare, and the Civil Rights Act of 1964. He served as postmaster general (1965–68) before managing Robert Kennedy's presidential campaign; after the second Kennedy assassination he became presidential campaign director for Hubert Humphrey (1968).

He twice (1968–69, 1970–73) chaired the Democratic National Committee; it was his office that Republican campaign workers burglarized in 1972, leading to the Watergate scandal. In 1975 he became commissioner of the National Basketball Association, a post he held until 1984.