O'Neil was born in Boulder, Colorado and attended Dennison Elementary. His family hired him an acting coach after he did well in school oral interpretation classes. O'Neil subsequently won five silver and one bronze medals at amateur acting competitions during the summer of 2005 and was taken on by Cutler Management, who sent him to study acting. Shortly after that, he was cast in Sihirli kutu (2007), which was his second acting audition.

O'Neil lives with his family in the foothills outside of Denver, Colorado, near the Rocky Flats Plant. His favorite video game is The Lord of the Rings: The Battle for Middle-Earth II (2006). As of March 2007, he is in sixth grade and attends school on set while filming. He has an older brother, Tyler, and comes from a "very Christian" family.