In World War I many Diocesan parishioners joined the armed services and several priests became chaplains.

O'Leary founded a convent in 1916 when the Sisters of St. Martha of Prince Edward Island was established.

The Charlottetown Hospital, which had been backed by Bishop Peter McIntyre in the 19th century continued to grow as a new maternity department opened in 1918 and a school of nursing was established in 1920.

The hospital was under the supervision of the Grey Nuns of Quebec, whose presence had been felt in the Diocese of Charlottetown for many years.

After seven years as Bishop of Charlottetown, O'Leary was appointed Archbishop of the Archdiocese of Edmonton, in the Western Canadian region.