Father Ouellet returned to America at the start of the Civil War, and he accepted a military chaplaincy in the 69th New York Regiment, a predominantly Irish-American unit. Described as a “short, spare, active, lively man,” Ouellet served for all four years of the war, through some of the roughest fighting. One historian writes of his army career:
He did untold good and won for himself golden opinion from officers and men, Protestant and Catholic. During battle he took his place on the firing line, in the most exposed spot, to be nearer to those who fell, and give them the promptest aid. Needless to say, such bravery won him respect and authority, an authority which was invaluable in the exercise of his ministry. He would brook no interference with his duties as chaplain, and no one ever tried it a second time.
After the Battle of Malvern Hill in 1862, Father Ouellet walked among the dead and dying, asking if anyone was Catholic and wanted absolution. A mortally wounded Confederate officer replied: “No, but I would like to die in the faith of any man who has the courage to come and see me in a place such as this.”
After the war, Father Ouellet worked in parishes in New York and Canada. In 1879, he began a fourteen-year ministry to Native Americans out West. Poor health brought him back to Montreal in 1893, and he died there the following year at age seventy-five.