O’Brien grew up participating in several sports, including football, baseball, basketball and swimming. His father, Paul, swam in college and his mother, Anna, played volleyball for the Swedish national team.
O’Brien’s favorite sport growing up was baseball. He played on several travel teams, often filling strong-arm positions such as pitcher, catcher and right field.
As an eighth-grader, he played linebacker on a Saddleback Valley Wolverines Pop Warner team that featured Tesoro quarterback Devon Modster and Santa Margarita quarterback KJ Costello.
It was around that time O’Brien began thinking about switching to quarterback. His neighborhood friend, ex-St. Margaret’s quarterback Josh Davis (now at Lafayette), encouraged him to train with his private passing coach, Steve Calhoun, and O’Brien took the advice.
O’Brien said one of the reasons he picked his hometown school of San Juan Hills was the freedom to play football and baseball. He played quarterback for the first time as a freshman and after his first spring of baseball, decided to focus solely on football.