Akili Arnold attended Mission Viejo High School in California, where he played defensive back and running back on the football team and also competed in track and field. He was named the 2018 County Defensive Player of the Year by the Orange County Register and the Offensive MVP of the South Coast League. During his senior season, he recorded 32 tackles and seven interceptions, while hauling in 53 passes for 870 yards and seven touchdowns. Arnold waws ranked among the top 100 players in the state of California by 247Sports. The three-star safety committed to Oregon State during his senior season.
Arnold totaled 17 tackles, including 0.5 tackles for loss while appearing in four games as a freshman in 2019. As a redshirt freshman in 2020, he had 30 tackles, including two for a loss and one forced fumble in seven games, including six starts. He appeared in 13 games and started five for the Beavers in 2021 tallying 31 tackles and one interception. As a redshirt junior in 2022, he tallied 12 tackles and one fumble recovery while appearing in 13 games and starting two.
Arnold had his best season in 2023, starting all 11 games and collecting 60 tackles and two interceptions. With one year of eligibility remaining, Arnold decided to enter the transfer portal and signed with USC.