O'Toole studied at the Glasgow (Scotland) School of Art and then became active in Irish politics. He serve as captain of the Irish Republican Army, leading the rebels of the Irish Civil War of 1922. He came to the United States in 1928 and studied at the Art Students League and at the National Academy of Design School under Kroll, Neilson, Olinsky and Covey (1928-1934). In 1934 he won a Pulitzer Scholarship and spent three years in the British Isles, France and Switzerland. He studied etching in Paris under Stanley W. Hayter and painting under Andre L'Hote and with Hans Hoffman.
He taught drawing and painting at Fitch Junior College (1938-41), was a lecturer in drawing a Columbia University (1948-49) and was an instructor and later head of the School of Design at Wilkes College (1950-59). In 1941 he married Kelsey Frazier. He was proposed to the National Academy by Hobart Nichols.
