Reverend O’Reilly was a professor of religious knowledge at St. Michael’s College of the University of Toronto where he himself had been a student of Rev. Dr. Peter Guilday. He also gave a course on the medieval Church to girls at St. Joseph’s College. Later, he taught at St. Augustine’s Seminary in Toronto. He was a member of the American Catholic Historical Association and first vice-president of the English section of the Canadian Catholic Historical Association. Among the articles he wrote for various Catholic periodicals was "Quebec's Most Interesting Building," in which he praised Quebec City’s Hôpital général; in the same article he called Ramsay Traquair the "greatest living authority on the architecture of its [Quebec's] old buildings and manoirs." He also wrote “Mére Marie de l’Incarnation,” “Lighting the Lamp” and “The Utopia of St. Thomas More.” In his retirement, he lived in Peel, Ontario.
