[2] He subsequently studied for a DPhil at Nuffield College, Oxford as the first student to be supervised by the psephologist David Butler.
His thesis was published as The Elimination of Corrupt Practices in British Elections, 1868-1911 (Clarendon Press, 1962) and in the same year he was appointed lecturer at Queen's University Belfast in Northern Ireland.
[2] He was the first Catholic to hold such a chair at the university[2][4] and explained that, when he was appointed in 1960, he saw Queen's as part of the Unionist establishment.
[3] According to an obituary written by Bernard Crick, O'Leary suffered from alcoholism, which resulting in him often being absent from the university and colleagues having to cover for him.
A Study of Belfast Politics, 1613-1970 (with Ian Budge, Macmillan, 1973), The Northern Ireland Assembly, 1982-1986: A Constitutional Experiment (with Sydney Elliott and R.A. Wilford, Hurst, 1988) and Controversial Issues in Anglo-Irish Relations, 1910-1921 (with Patrick Maume, Four Courts, 2004).