[2] Failing health due to tuberculosis forced O'Connor to return to Toronto in 1863 and allowed him to be ordained to the priesthood before the canonical age of 25.

[4] At age 22, O'Connor was ordained a priest on 8 December 1863 by Bishop John Joseph Lynch at St. Mary's Church in Toronto.

[2] During his 20 years at the helm of Assumption, he turned the college around from a state of disrepair by tripling the number of students, expanding the curriculum, and adding two wings to the building.

[5] He received his episcopal consecration on the following 19 October from Archbishop Walsh, with Bishops John Samuel Foley and Thomas Joseph Dowling serving as co-consecrators, at St. Peter's Cathedral in London.

[7] He avoided significant expansion of the Diocese of London during his tenure, having inherited debt from the construction of St. Peter's Cathedral and overseeing a decline in the local Catholic population from 70,000 in 1890[8] to 60,000 in 1899.

[6] With his academic background, O'Connor made a strong push for the certification of separate school teachers and tried to guarantee an equal education for Catholic children.

[2] He also prohibited women from serving in church choirs, applied a strict implementation of Pope Pius X's Tra le sollecitudini that discouraged more modern liturgical music, banned parishes from holding picnics, and refused to allow the Knights of Columbus to expand into the archdiocese on the grounds that there were already too many Catholic societies.