He attended D'Evelyn Junior/Senior High School as a seventh grader. He attended Denver Academy.
His family hired an acting coach for him after he did well in school oral interpretation classes. Shortly thereafter, in what was only his second acting audition, he was cast in The Last Mimzy. O'Neil had recently attended St. John's Military School in Salina, Kansas and graduated from Wasatch Academy in Mt.
Pleasant, Utah, he then lived in Laurel, Maryland while he attended the Rochester Institute of Technology.