Born in Hanover, Pennsylvania, O'Neill was in the United States Navy Reserve from 1948 to 1953.
He was a law clerk to Judge Herbert Funk Goodrich of the United States Court of Appeals for the Third Circuit from 1953 to 1954, and to Justice Harold Hitz Burton of the Supreme Court of the United States from 1954 to 1955.
Prior to his appointment to the United States District Court for the Eastern District of Pennsylvania, he was in private practice at Montgomery, McCracken, Walker & Rhoads, LLP in Philadelphia, Pennsylvania from 1956 to 1983, where he was a partner and chair of the litigation department.
[1] [2] In 1976, O'Neill served as the Chancellor of the Philadelphia Bar Association.
[3] On June 21, 1983, O'Neill was nominated by President Ronald Reagan to a seat on the United States District Court for the Eastern District of Pennsylvania vacated by Judge Edward R. Becker.