O'Sullivan studied at the Dublin Metropolitan School where he was awarded a teacher-in-training scholarship, and later studied painting in Paris in the 1920s. From 1926 onward, he showed work regularly at the Royal Hibernian Academy annual exhibitions and in 1928 was appointed an Associate member. In 1931, he was elected a full member of the RHA.
O'Sullivan was an accomplished draughtsman, known for his portrait drawings of Irish personalities, including James Joyce, Maude Gonne & Eamon de Valera. Throughout his career, he contributed illustrations for several Dublin magazines and newspapers including the Capuchin Annual. His portrait paintings and drawings are held in most of Ireland's main collections including the National Gallery, all of the municipal collections, Irish Museum of Modern Art, and the National Museum.
