High school career
Onuaku attended Riverdale Baptist School where he averaged 12.4 points, 12.7 rebounds, and 5.5 blocks as a senior, leading Riverdale to a 30–9 record and the Capital Beltway conference title.
When Onuaku graduated, he was considered the 74th best prospect by Rivals.com, 75th by ESPN, and was rated as the seventh-best center in the nation by Scout.com.
Onuaku played two seasons of college basketball for the University of Louisville between 2014 and 2016. In his sophomore season, he averaged 9.9 points, 8.5 rebounds, and 2.0 blocks, earning All-ACC Defensive Team and All-ACC honorable mention honors and posting 11 double-doubles.
In May 2016, Onuaku announced he would enter the NBA draft.
Houston Rockets (2016–2018)
On June 23, 2016, Onuaku was selected by the Houston Rockets with the 37th overall pick in the 2016 NBA draft and later joined them for the 2016 NBA Summer League. On July 20, 2016, he signed with the Rockets. He made his NBA debut on December 26, 2016, coming on in the fourth quarter and recording six points and three rebounds in a 131–115 win over the Phoenix Suns. He hit a pair of free throws in the game with his underhanded free-throw action. During his rookie season, Onuaku had multiple assignments with the Rio Grande Valley Vipers, the Rockets’ D-League affiliate.
On August 2, 2018, Onuaku was traded from the Rockets to the Dallas Mavericks for the rights to forward Maarty Leunen. He was waived four days later.[
On September 4, 2018, Onuaku signed with the Portland Trail Blazers. On October 13, 2018, he was waived by the Trail Blazers.
On October 20, 2018, Onuaku was selected with the second overall pick in the 2018 NBA G League draft by the Greensboro Swarm.