On December 11, 1939, O'Hara was appointed by Pope Pius XII as an auxiliary bishop of the United States Military Ordinariate, which served the spiritual needs of the nation's armed forces, as well as the titular bishop of Milasa. He received his consecration as a bishop on January 15, 1940, from Archbishop Francis Spellman, with Bishops John F. Noll and Joseph Ritter serving as co-consecrators, in Sacred Heart Church in Indiana. A devotee of the Blessed Virgin Mary, he selected as his episcopal motto: "Following her, you will not go astray."
