Dr. O'Neill has been a practicing cardiologist since 1990. Health care leadership experience includes Director of the Cardiac Catheterization Laboratory of the Victoria General Hospital and later the QEII Health Sciences Centre in Halifax. He went on to become Chief of the Division of Cardiology at the QEII Health Sciences Centre. In 2008 he was recruited as Division Director for the University of Alberta, Division of Cardiology and for the Mazankowski Alberta Heart Institute. In 2012, he became Senior Medical Director for the Cardiovascular Health and Stroke Strategic Clinical Network for Alberta Health Services, and in 2015 became the Associate Chief Medical Officer for all Strategic Clinical Networks in the province of Alberta. He served on Council of the Canadian Cardiovascular Society (CCS) from 1995 until 2000, and on its Executive from 2008-2012, serving as President of the Society between 2010-2012. He served as Chair of the Access to Care Committee for the CCS, and was instrumental in setting the targets for cardiovascular services and procedures, which have been adopted nationally. He also chaired the CCS' Quality Initiative, which established common data-definitions for cardiovascular registries in Canada, as well as the initiative to define national quality indicators for cardiovascular conditions. He also served as Chair of the Quality Collaborative for the Canadian Medical Association.
He has been involved in many cardiovascular reviews. He has reviewed the Cardiology Division at McMaster University, the Cardiology Division at St. Michaels Hospital in Toronto, as well as the Cardiac Sciences Program at Sunnybrook Health Sciences Centre in Toronto. In Alberta, he chaired the review for cardiac surgery wait times and proposed mitigating solutions. He also co-chaired the task force to examine expansion of advanced cardiac services including standalone percutaneous coronary intervention to the central zone of Alberta Health Services.