Dr O’Sullivan received his medical degree from the National University of Ireland, Galway. After internal medicine training, he was accepted on the Cardiology Specialist Training Scheme of the Royal College of Physicians of Ireland, receiving his CSCST in Cardiology in 2017. During this time, he received a Molecular Medicine Ireland Fellowship to undertake a PhD at University College Cork, completed in 2012.
He was awarded an overseas training fellowship by the Irish Cardiac Society in 2012 to train at Massachusetts General Hospital (MGH) and Harvard Medical School. From 2012–2016, he was also a postdoctoral fellow at MGH, studying functional genomics and metabolomics, and worked with the Framingham Heart Study and Broad Institute of Harvard and MIT. He joined Heart Research Institute in 2016 through the Sydney Cardiovascular Research Fellowship.