[2] Following his return to the United States later that year, he served as a curate at St. Mary's, Windsor Locks until 1924, when he was transferred to St. Thomas Aquinas, Fairfield.
[2] He received his episcopal consecration on the following May 14 from Archbishop Amleto Giovanni Cicognani, with Bishops Maurice F. McAuliffe and Joseph Edward McCarthy serving as co-consecrators.
The new structure was consecrated on 15 May 1962 by Auxiliary Bishop John F. Hackett; due to illness, Archbishop O'Brien was unable to preside.
[4] He later joined his fellow Catholic bishops in Connecticut in opposing a bill that would have permitted abortions for pregnancies resulting from rape.
[4] After twenty-three years as head of the Hartford Archdiocese, O'Brien resigned on November 20, 1968; he was appointed Titular Archbishop of Uthina on the same date.