He started playing basketball at age 14 and stood under 6 ft 3 in (1.91 m) as a high school freshman.

[2] During his freshman season at Shawnee State, Onu set a program single-game record with 11 blocks against the University of the Cumberlands.

He was an Honorable Mention National Association of Intercollegiate Athletics (NAIA) All-American, First Team All-MSC and MSC Defensive Player of the Year.

[8] After the season, Onu declared for the 2021 NBA draft while maintaining his college eligibility and entering the transfer portal.

[19] On April 5, 2022, Onu signed with the Niagara River Lions of the Canadian Elite Basketball League.

[22] On January 22, 2023, Onu was traded to the Santa Cruz Warriors in exchange for Tremont Waters.

[27] Onu signed a contract with Legia Warsaw of the Polish Basketball League (PLK).