He soon received his commission as a second lieutenant of Infantry, and served until 1940, when his unit was federalized for army service during World War II.

O'Hara advanced through the ranks to lieutenant colonel during the war as commander of 2nd Battalion, 106th Infantry Regiment and took part in several battles in the Pacific theater.

After the war, he pursued a civilian career with the Universal Match Corporation while continuing to serve in the National Guard.

[8] O'Hara was named for his maternal grandfather Almerin W. Cartwright (1855–1919), a prominent store owner and postmaster of Roxbury, New York.

[14] O'Hara later pursued a civilian career with the Universal Match Company of Buffalo, New York; as a manager and executive, he had responsibility at different times for departments including sales and production.

[18] Additionally, O'Hara was a member of the Freemasons, Rotary International, American Legion, and Veterans of Foreign Wars.

[6] In 1941, he completed the Communications Course for Infantry officers, and in 1943 he graduated from the United States Army Command and General Staff College.

[19] O'Hara had been promoted to lieutenant colonel by the end of the Second World War II, and in September 1945 he was released from active duty.

[6] He was then commissioned in the United States Army Reserve, where he served until April 1947, when he rejoined the New York National Guard.

[6] In April 1957, O'Hara was appointed to command the 27th Armored Division, and in August 1957 he received promotion to major general.

[b][6] In 1966, O'Hara attended the Reserve Components Senior Officer Course at the United States Army War College.

[27] In addition, he carried out assignments at the national level, including member of the Reserve Forces Policy Board.

[33] During the Attica Prison riot in September 1971, Governor Nelson Rockefeller dispatched O'Hara to the scene as his personal representative.