A large portion of his compositions were based in folklore and he also arranged and harmonized roughly 400 French and Canadian folksongs; many of which were written for his collaborations with Charles Marchand and the Alouette Vocal Quartet.
[1] Born in Ottawa, O'Brien was a pupil of Amédée Tremblay with whom he began studying both the piano and the organ as a young teenager.
Among his pupils were Joseph Beaulieu, Lionel Daunais, Hector Gratton, Jacques Labrecque, Allan McIver, Lucien Sicotte, and Albert Viau.
The quartet gave regular concert tours in the United States, mainly performing in New York City, Washington D.C., Detroit, Cleveland, Chicago, and Philadelphia.
[2] A devout Roman Catholic, O'Brien became interested in pursuing a religious life and entered the Benedictine monastery at Saint Benedict Abbey, Quebec as a novitiate in 1945.