O'Neill grew up in Boulder, Colorado and attended Fairview High School. In his senior year of high school, O'Neill was named the Colorado's player of the year by Gatorade and selected to the All-Colorado boys soccer team. O'Neill played basketball in high school and was named to the second-team all-5A team for his performances during his senior year. Although he committed to the University of Virginia, O'Neill ultimately signed with his hometown team the Colorado Rapids at the age of 18.
