O'Brian was born in Buffalo, New York in 1874. He received the A.B. degree from Harvard College in 1896 and the L.L.B. degree from Buffalo Law School in 1898. In February 1909, O'Brian was appointed by President Theodore Roosevelt to serve as United States Attorney for the Western District of New York. He continued to serve in this position through the administrations of President Taft and President Wilson. During World War I, O'Brian served as Head of the War Emergency Division in the United States Dept. of Justice where he was responsible for prosecuting cases of espionage and sabotage. At the end of World War I, O'Brian returned to Buffalo to practice law. In 1929, President Hoover appointed O'Brian to serve as Assistant Attorney General of the Anti-Trust Division at the United States Department of Justice where he was responsible for arguing more than 15 cases before the United States Supreme Court. He was retained by the Tennessee Valley Authority in 1935, eventually winning the case that challenged the creation of the Authority. In 1941, President Franklin Roosevelt appointed O'Brian to serve as General Counsel of the War Production Board. From 1945 until his death, at age 98, O'Brian practiced law in Washington, D.C.

O'Brian's service to the University at Buffalo Law School began soon after his graduation in 1898. From 1907 through 1921, O'Brian was a volunteer instructor in Insurance Law. From 1931 and continuing for the next seventeen years, O'Brian served as member of the Board of Regents of the University of the State of New York. O'Brian served the Law School in many capacities and was the principal speaker at the October 21, 1949 dedication of 77 West Eagle Street, a former home of the UB Law School. On May 6, 1963, O'Brian received the Law School's first Distinguished Alumnus Award for Public Service. At the formal dedication of O'Brian Hall in April 1974, a portrait of O'Brian, painted by Virginia Cuthbert, was presented to the Law School by Manly Fleischmann, a SUNY trustee and Mrs. Kellogg Mann, the eldest daughter of O'Brian. This portrait currently hangs on the second floor of the Law School Library.

As recognition of his public service and service to the Law School, the home of the U.B. Law School, "O'Brian Hall", was named in his honor in 1973 when it first opened on the Amherst campus.

Information excerpted from: Schaus, Robert and James Arnone, "University at Buffalo Law School, 100 Years 1887-1987, A History" John Lord