A graduate of Notre Dame, David J. O'Brien earned his Ph.D. in history at the University of Rochester in New York. He specializes in the history of Catholicism in the United States. David O'Brien served as Professor of History and Loyola Professor of Catholic Studies at the College of the Holy Cross for almost forty years, and taught American Catholic history and Catholic social and political thought. His long teaching career at Holy Cross was also accompanied by national recognition and acclaim. A past president of the American Catholic Historical Association, Professor O'Brien was also the founding director of the Center for Religion and Ethics and Culture at Holy Cross. After retirement he accepted a position as University Professor of Faith and Culture at the University of Dayton.

A frequent lecturer, Professor O'Brien is the recipient of many honors and awards for his work, including the Theodore M. Hesburgh Award for distinguished service to Catholic higher education from the Association of Catholic Colleges and Universities and the University of Dayton Marianist Award for outstanding contribution to scholarship and intellectual life. He has also been at the forefront of social causes and activism since the Vietnam War protests. One of the first scholars to specialize in American Catholic history, O'Brien is widely published.

His publications include American Catholics and Social Reform: The New Deal Years (Oxford, 1972); The Renewal of American Catholicism (Oxford, 1972); (co-editor), Renewing the Earth: Catholic Documents on Peace, Justice and Liberation (Doubleday, 1977); Faith and Friendship: Catholicism in the Diocese of Syracuse, 1886-1986 (Catholic Diocese of Syracuse, 1987); Public Catholicism (Macmillan, 1988); Isaac Hecker (1992).

In 2005, the Parish Pastoral Council of Our Lady of Sorrows Parish established the Rev. Robert W. Bullock Lecture in memory of its beloved former pastor. This annual lecture is delivered in the spring of each year by a distinguished scholar or religious leader. This event honors the memory of Father Bullock and provides an ongoing forum in which the people of Our Lady of Sorrows Parish and of the Sharon community can be in dialogue with speakers who share Father Bullock's ideals and aspirations.