O'Brien was born in New York City, the son of Lisa (née Rhodes), a former actress who also ran an acting school, and Patrick O'Brien, a camera operator. He grew up in Springfield Township, New Jersey, until the age of twelve, when he and his family moved to Hermosa Beach, California. His father is of Irish descent, and his mother is of Italian, English and Spanish ancestry. After graduating from Mira Costa High School in 2009, he considered pursuing sports broadcasting and possibly working for the New York Mets. At 14, O'Brien began posting original videos onto his YouTube channel. With the videos developing, a local producer and director approached him about doing work for a web series while in his senior year of high school. While working on the webseries, O'Brien met an actor who connected him with a manager. He had planned to attend Syracuse University as a sports broadcasting major, but decided to pursue an acting career.