[3] He received his episcopal consecration on May 21, 1929, from Cardinal Dougherty, with Bishops John MacGinley and Thomas O'Reilly serving as co-consecrators, at the Cathedral Basilica of Saints Peter and Paul in Philadelphia.
[6] The cathedral was built on the former site of Ku Klux Klan gatherings, and O'Hara even invited Imperial Wizard Hiram Evans to the dedication.
[8] O'Hara was considered a leader in church efforts to improve race relations,[9] launching a seven-point social and racial program in the 1930s, calling for aid to African American children and heightened awareness of rural issues.
[5] In addition to his role as diocesan bishop, O'Hara was named regent of the apostolic nunciature to the Socialist Republic of Romania in Bucharest, on February 19, 1947.
O'Hara accused the Romanian Government of terrorism, insisting, "Our interest was solely the welfare of 3,000,000 Catholic people in Romania.
[3] In 1953, Paul Blanshard, author and critic of the Catholic Church, petitioned the U.S. Department of State to revoke O'Hara's American citizenship.
Blanshard claimed that O'Hara was violating the 1950 McCarran Internal Security Act by serving as an agent of a foreign power (the Vatican).
The State Department dismissed Blanshard's petition[13] On June 8, 1954, O'Hara was named apostolic delegate to Great Britain.
[10] In May 1958, O'Hara was admitted to St. Anthony at Cheam Hospital in London with a severe case of bronchial pneumonia after returning to the United Kingdom from a foreign trip.