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.

[4] He has also advised the United Nations, the European Union and the UK's Department for International Development.

[4] In both his own writings and those with John McGarry, O'Leary has long backed consociationalism as a method of conflict management for Northern Ireland, and is supportive of the Good Friday Agreement.

[2][6] In 2009, a book entitled Consociational Theory: McGarry and O'Leary and the Northern Ireland Conflict was published, edited by Rupert Taylor.

Recent books include Power-Sharing in Deeply Divided Places (co-edited with J. McEvoy), Divided Nations and European Integration (co-edited with T.Mabry, J.McGarry and M. Moore), and Courts and Consociations (coauthored with Christopher McCrudden).