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.

[3][4] 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.

In his role as the federal government's principal attorney in western New York, O'Brian in 1913 filed an antitrust lawsuit alleging that the Eastman Kodak Company was maintaining an unlawful monopoly on photographic films and equipment.

He was the nominee of both the Progressive and Citizens parties, netting 23,757 votes to Democrat Louis Fuhrmann's 30,219 and Republican Thomas Stoddart's 13,447.

[12] To control unfettered and inconsistent enforcement of the statutes, O'Brian forbade local federal prosecutors from filing charges of espionage, sedition, or treason without approval from his Division in Washington.

In the words of historian Theodore Kornweibel, O'Brian and Bettman "attempted to curb the most egregious violations of civil liberties.

[15][16] While in charge of the War Emergency Division, O'Brian hired J. Edgar Hoover, who had just finished his legal education in Washington.

Impressed with Hoover's diligence, O'Brian promoted him to Special Agent in charge of the Division's Alien Enemy Bureau.

Instead, O'Brian served in the Hoover administration from 1929 to 1933 as the Assistant Attorney General in charge of the Antitrust Division of the Justice Department.

[24] In the later 1930s, the Tennessee Valley Authority hired O'Brian as counsel to defend legal challenges to the constitutionality of the federally-owned corporation.

[29] When his government service concluded in the last months of World War II, O'Brian joined the Washington, D.C., law firm Covington & Burling on January 1, 1945.

[34] More than a year later, he was still arguing for clients before the federal circuit court of appeals in Washington, D.C.[35] In 1955, O'Brian delivered the Godkin Lectures at Harvard University, entitling his series "Security in an Age of Anxiety.