Following undergraduate training at Cambridge University, Dr O’Sullivan undertook postgraduate medical training at The Royal Brompton National Heart & Lung Hospital and the Oxford University Teaching Hospitals. He returned to Cambridgeshire in 1996 and over the next seven years trained in all aspects of cardiology, with particular emphasis on interventional cardiology, at Papworth and Addenbrooke's Hospitals. During this period he was awarded a PhD, and the British Cardiac Society Young Research Worker's Prize, for research into mechanisms of coronary stent failure. From 2003-2004, he worked in the Department of Interventional Cardiology at University of British Columbia, Vancouver and gained extensive experience in advanced interventional techniques and new strategies for the treatment of coronary disease. Following his return Dr O’Sullivan was appointed Consultant Cardiologist at Papworth and Addenbrooke's Hospitals. In this position he provides secondary and tertiary centre care to patients with general cardiac problems (valve disease, heart failure, heart rhythm problems), in addition to those with coronary disease. Dr O’Sullivan became Director of the Addenbrooke's Cardiology Department in 2006.

Echocardiography and exercise testing are available at the time of consultation Dr O’Sullivan’s Spire Cambridge Lea Hospital clinic.