Dr O’Donnell qualified at the National University of Ireland, Galway, in June 1978 and completed his General Practice training in Portsmouth in 1982.
He worked in tertiary referral medicine in Baghdad, Iraq from 1984 to 1988 followed by a cardiology research post at Wexham Park Hospital (UK) involving the first use of thrombolysis in acute myocardial infarction.
This led to a career in the pharmaceutical industry involving novel treatments for ischaemic heart disease. He continued as a GP and was involved in the development of new medicines up to 1998 at which time he returned to full-time general practice in Slough and had a key interest in clinical governance and education in primary care. In 2007 he became the Slough Locality Lead and shortly after in 2008 he became Chairman of the Clinical Executive Committee at Berkshire East PCT.
He was appointed Chair of NHS Slough CCG in 2013, and Slough Locality Lead for NHS East Berkshire CCG in 2018, when three local CCGs merged. In July 2014 he led the first 7-day in-hours GP service provision in England in Slough. He was involved in drawing up initial plans for the NHS Commissioning Assembly, later to become NHS Clinical Commissioners.