O'Leary is the author of numerous influential books about the Northern Ireland conflict, many of them co-authored with John McGarry, whom he met when they both attended Saint MacNissi’s College. McGarry and O'Leary's "Policing Northern Ireland: Proposals for a New Start" (Blackstaff Press, 1999) is considered to have had a significant influence on the work of the Independent Commission on Policing for Northern Ireland. He has been an international advisor to the Kurdistan National Assembly, responsible for advising on the constitutional reconstruction of Iraq and Kurdistan, with special responsibility for federal arrangements and electoral laws. He has also advised the United Nations, the European Union and the UK's Department for International Development. Previously, he was a policy advisor to the British Labour Party, and political advisor to Mo Mowlam and Kevin McNamara during their respective spells as Shadow Secretary of State for Northern Ireland. In the course of his career he has supervised more than thirty doctoral dissertations, including those of political scientists Stefan Wolff, Katharine Adeney, Lori Thorlakson, Shelley Deane and Tristan James Mabry.