He received a Bachelor of Arts degree from the University of California at Berkeley in 1973, a Master of Public Administration degree from Golden Gate University in 1976, and a Juris Doctor from the University of California, Hastings College of the Law in 1979.

Between 1986 and 1992, he was an adjunct professor at San Joaquin College of Law in Clovis.

He was a judge on the Fresno County Superior Court for the State of California from 1990 to 1999.

From 1999 to 2007, O'Neill served as a United States magistrate judge of the United States District Court for the Eastern District of California.

On January 9, 2007, O'Neill was nominated by President George W. Bush to a seat on the United States District Court for the Eastern District of California vacated by Oliver Winston Wanger.