Admiral Sir William Donough O'Brien (13 November 1916 – 19 February 2016) was a British military leader. He was the Commander-in-Chief of the Western Fleet. He was then appointed Naval Secretary in 1964. He was appointed Flag Officer Aircraft Carriers in 1966, Commander-in-Chief of the Far East Fleet in 1967 and Commander-in-Chief of the Western Fleet in 1970. He retired in 1971.