O'Brien was born in New York City. His parents are Lisa O'Brien, a former actress and Patrick O'Brien, a camera operator. He grew up in Springfield Township, New Jersey. When he was twelve his family moved to Hermosa Beach, California. His father has Irish ancestry. His mother has Italian, English and Spanish ancestry. When he graduated from Mira Costa High School in 2009, he planned to attend Syracuse University as a sport broadcasting , but he decided to move to Los Angeles to pursue an acting career. At fourteen, O'Brien began to post videos on his Youtube channel. Because of that, a local producer and director discovered him.