O'Hara received his early education in that city and later attended Georgetown College in Washington, D.C.[2] In 1834, having decided to become a priest, O’Hara applied to Bishop Francis Kenrick for acceptance as a seminarian for the Diocese of Philadelphia.

[6] He held that position for thirteen years (1843-1856), during which time he was sometimes assigned to help at other parishes in Philadelphia and around the state (including Chambersburg and Honesdale).

[4] In addition to his pastoral duties, O’Hara was named rector of St. Charles Borromeo Seminary in 1853 by Bishop John Neumann.

Bishop Neumann increased the heavy workload of the rector-professor-assistant in 1856, when he promoted O’Hara to full pastor of St. Patrick's Parish.

[7] O’Hara was named vicar general of the diocese in 1860 by Bishop James Wood, leaving the seminary but remaining at St. Patrick's Church.

Early in his tenure, O'Hara attended the First Vatican Council in Rome (1869-1870), where he voted in favor of papal infallibility.

Given his advanced age, he requested a coadjutor bishop to assist and eventually succeed him, and he received Michael John Hoban in 1896.