Justice O’Callaghan was educated at the University of Melbourne and Yale University.
Between 1982 and 1983, he was Associate to the Hon. Justice Keely of the Federal Court of Australia, before moving to the United States to practise at Sullivan & Cromwell in New York (1985) and Wiggin & Dana in Connecticut (1998-2000). He signed the Roll of the Victorian Bar in 1986 and was appointed Senior Counsel in 2003. While at the Bar, he practised widely in federal and state jurisdictions, appearing in trial and appellate matters involving corporations law, trade practices law, revenue law, franchising law, equity, mining, energy and resources law and constitutional law. He was the Senior Vice-President of the Victorian Bar Council (2015-2016), Chair of the Victorian Bar Education and Professional Development Committee (2013-2016), Member and Chair of the Readers’ Course Sub-Committee (2010-2016) and a director of Barristers’ Chambers Limited (2013-2015).
Between 1996 and 1998 he was a Senior Research Affiliate and Visiting Lecturer at Yale Law School and he is currently a Senior Fellow at Melbourne Law School.