After receiving an early education at public and religious schools, O'Hara attended the Detroit College of Law, graduating in 1908.

[1] After graduating in 1908, O'Hara started practicing law in Menominee, Michigan immediately.

Before his time as Michigan Auditor General, O'Hara had 27 years of law experience in the Upper Peninsula of Michigan, where he gained a specialization in corporate and criminal practices.

Upon the death of Michigan Auditor General John K. Stack Jr. in office, O'Hara was appointed to the position by Michigan Governor Frank Fitzgerald on January 21, 1935.

John was a member of the Knights of Columbus, and served for two years as a state deputy for the order.