James P. O'Hara was born in Detroit, Michigan and raised mainly in Utah and Nebraska. He earned a Bachelor of Arts degree from the University of Nebraska in 1977. He attended the Creighton University School of Law, where he was elected to the Moot Court Board and served on the Editorial Staff of the Creighton Law Review. He earned his juris doctorate degree, with honors, in 1980.

Following law school, O'Hara served a two-year judicial clerkship with U.S. District Judges Robert V. Denney and C. Arlen Beam in the District of Nebraska. From 1982 until his appointment to the federal bench in 2000, O'Hara was in private practice with a large Kansas City law firm, for the first five years as an associate and then as a partner, primarily representing clients in commercial litigation cases in Kansas and Missouri. He was appointed U.S. Magistrate Judge in 2000, initially serving in Topeka and, since 2003, in Kansas City. At the time of O'Hara's appointment to the bench, he was serving on his law firm's executive committee and as managing partner of its office in Overland Park, Kansas.

While in private practice, O'Hara served on the Ethics and Grievance Committee of the Johnson County Bar Association, on the Kansas Board for Discipline of Attorneys, on the Bench-Bar Committee of the U.S. District Court for the District of Kansas, and on the boards of civic and church organizations.

Since joining the bench, Judge O'Hara has remained active in several bar and continuing legal education endeavors, including the Earl E. O'Connor American Inn of Court, the Board of Editors of the Journal of the Kansas Bar Association, and teaching Trial Advocacy as a member of the adjunct faculty at the University of Kansas School of Law.