In his junior and senior seasons, Okongwu led Chino Hills to two more state titles, earning back-to-back California Mr. Basketball honors.

[3][4] In his first year, he was teammates with brothers Lonzo, LiAngelo, and LaMelo Ball, who helped elevate the team into the national spotlight.

[5] His team ranked number one in the country with a 35–0 record and captured the California Interscholastic Federation (CIF) Open Division state title.

[6][7] Okongwu shared MaxPreps National Freshman of the Year honors with his teammate, LaMelo Ball, after averaging 7.9 points, 7.2 rebounds, and 3.1 blocks per game.

[17] Okongwu finished his freshman season with offers to play college basketball for UCLA and USC.

[20] In his collegiate debut on November 5, 2019, he recorded 20 points, 13 rebounds and a school-record eight blocks to lead the Trojans to a 77–48 victory over Florida A&M.

[23] On December 1, Okongwu recorded 27 points on 12-of-14 shooting, 14 rebounds and three blocks in a 77–62 victory over Harvard at the Orlando Invitational.

[31][32] On March 25, 2020, Okongwu announced that he would enter the 2020 NBA draft and forgo his final three years of college basketball eligibility.

[41] Highly influenced by his older brother, Nnamdi, Onyeka's passion for basketball began to grow.

Okongwu's older brother, Nnamdi, also played basketball for Chino Hills High School.

[42] In 2014, Nnamdi sustained a brain injury in a skateboarding accident and later died after spending three days on life support.

[3] In January 2020, Chino Hills High School retired the number in honor of his late brother.