He is a permanent member of the Pontifical Academy of St. Thomas Aquinas, appointed by Pope Benedict XVI in 2010. O'Callaghan is a past President of the American Catholic Philosophical Association. His areas of scholarly interest include Medieval Philosophy, the philosophy of St. Thomas Aquinas, and Thomistic Metaphysics and Ethics. He is the author of the book Thomistic Realism and The Linguistic Turn: Toward a More Perfect Form of Existence (2003, republished 2016), and has published more than twenty scholarly papers and book chapters. In 2016, he presented a paper at the "Disability and the Face of Mercy" conference sponsored by the Center for Ethics and Culture as part of the Jubilee Year of Mercy hosted at the Vatican.