John Lord O'Brian (October 14, 1874 – April 11, 1973) was an American lawyer who held public offices in the administrations of five U.S. presidents between 1909 and 1945. O'Brian has been recognized by scholars for his commitment to civil liberties. At the time of O'Brian's death at the age of 98, Chief Justice Warren Burger described him as the "dean" of the bar of the Supreme Court of the United States.