His service included a tour of the Vietnam War as an infantry captain with the 5th Battalion, Royal Australian Regiment from 1966 to 1967, during which he was Mentioned in Despatches.
[2] In 1965, he completed his Doctor of Philosophy thesis at the University of Oxford on "The Relationship between the German Army and the Nazi Party, 1933–9" under the direction of one of his predecessors as Chichele Professor of the History of War, N. H. Gibbs.
The research for O'Neill's thesis formed the basis for his first book, The German Army and the Nazi Party, 1933–1939, published in 1966.
He was head of the Strategic and Defence Studies Centre at the Australian National University from 1971 to 1982, senior fellow in international relations, 1969–77, professorial fellow, 1977–82, Australian official historian of the Korean War from 1970 until 1982, director of the International Institute for Strategic Studies (1982–87), and Chichele Professor of the History of War at the University of Oxford (1987–2001).
He was chair of the International Academic Advisory Committee at the United States Studies Centre, University of Sydney (2008–11).