He was born the son of merchant James O'Sullivan of Bandon, County Cork and was educated locally, before winning a scholarship to study chemistry at the Royal School of Mines, London.
After completing the three-year course he joined the Royal College of Chemistry as a student assistant to Professor August Wilhelm von Hofmann.
The following year Hofman's influence had secured O'Sullivan the post of assistant brewer and chemist to Messrs. Bass & Co. at Burton-on-Trent, Staffordshire, a major centre for English brewing.
They included On the Transformation Products of Starch (1872 and 1879); On Maltose (1876); On the Action of Malt Extract on Starch (1876); Presence of Raffinose in Barley (1886); Researches on the Gums of the Arabin Group (1884 and 1891); Invertase: a Contribution to the History of an Enzyme (with F. W. Tompson, 1890) and The Identity of Dextrose from Different Sources, with Special Reference to the Cupric Oxide Reducing Power (with A. L. Stern, 1896).
In 1885 he was elected a Fellow of the Royal Society, his nomination citation describing him as Distinguished as a chemist, especially for his researches on the Carbohydrates, which have thrown entirely new light on the constitution of this class of bodies and which are therefore of high importance in connexion with the chemistry of vital processes.