 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 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.