On his return to the United States, O'Connor passed through Ireland to recruit clergy for his new diocese, obtaining eight seminarians from Maynooth College and seven Sisters of Mercy from Dublin. He arrived in Pittsburgh in December 1843, where he found a diocese comprising 33 churches, 14 priests, and about 25,000 Catholics. To organize the new diocese, he held the first diocesan synod in 1844, and the same year he founded a girls' academy and orphan asylum, a chapel for African Americans, the "Pittsburgh Catholic" and St. Michael's Seminary. To serve the German immigrants in his diocese, he welcomed the Benedictine monks who founded Saint Vincent Archabbey in Latrobe, the first Benedictine monastery in the United States, and to further education he invited the Franciscan Brothers of Mountbellew in Ireland, who established the first community of religious brothers in the United States in Loretto.