 O'Hearn served in the Royal Canadian Navy during World War II, and a year after his discharge in 1945 was sent to New York as resident correspondent of the "Montreal Star".He was a founding member and first president of the United Nations Correspondents Association.While at the United Nations, O'Hearn co-wrote two books – published by the Canadian Institute for International Affairs in cooperation with the Carnegie Foundation – "United Nations' Struggle for Peace" and "Canada Stands Up."