He was educated at Eton College and Magdalen College, Oxford where he graduated MA.
O'Brien further studied at Imperial College London's Royal School of Mines, but worked in Kenya as a farmer and coffee planter from 1922 until 1936 when he was professionally engaged as a geologist in the mining industry by the Anglo-American Corporation of South Africa.
He was a geologist. He briefly served in Britain in World War I, which ended before he would have gone on active service or been promoted, as a Gunner in the Royal Field Artillery. He left in 1939 to serve in World War II with the Rifle Brigade, being commissioned in 1940 as Second Lieutenant and rising to Major in 1943. He was attached to the East African Intelligence Corps in Somaliland, Ethiopia and Madagascar, was mentioned in despatches in 1941, as well as wounded.
After demobilisation in 1946 he returned to the Anglo-American Corporation and worked for the company until he entered the British Colonial Service in 1954. He was employed on survey to the government of Northern Rhodesia(now Zambia), as senior geologist, becoming assistant director in 1957. He retired from the Colonial Service in 1959 but continued to work as a consultant geologist until 1967.
He is not to be confused with the British exploration geologist Christian O'Brien (1914-2001). After succeeding to his brother's peerage, he returned to the British Isles where he maintained Thomond House on the former ancestral estate of Dromoland Castle in Ireland, and a smaller home in England at Richard's Castle near Ludlow. He died in 1982.
