 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.O'Neill served as senior lecturer in history, Royal Military College, Duntroon, 1968–69.He was head of the Strategic and Defence Studies Centre, Australian National University in 1971–82, 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).