Osunniyi started playing basketball in eighth grade, after moving from Pleasantville, New Jersey to Somers Point.[1] He attended Mainland Regional High School in Linwood, New Jersey, joining the varsity team as a junior and starting for the first time in his senior year.[1][2][3] His coach, Dan Williams, considered him an NCAA Division III prospect before he grew three inches (7.6 cm) prior to his senior season. Osunniyi played a postgraduate season at Putnam Science Academy in Putnam, Connecticut. He led his team to the National Prep Championship, where he was named MVP of the title game.[4] He initially committed to playing college basketball for La Salle before switching his commitment to St. Bonaventure.[5] He turned down offers from traditional basketball powers, such as Syracuse and Georgetown.
College career
As a freshman at St. Bonaventure, Osunniyi averaged 7.5 points, 7.6 rebounds and 2.6 blocks per game. He led all NCAA Division I freshmen and ranked sixth in the nation in blocks. He was named to the Atlantic 10 All-Rookie and All-Defensive teams.[6] As a sophomore, Osunniyi averaged 10.8 points, 8.4 rebounds and 2.5 blocks per game. He earned Third Team All-Atlantic 10 and All-Defensive Team honors.[7] In his junior season, Osunniyi was a Second Team All-Atlantic 10 selection and Atlantic 10 Defensive Player of the Year.[8] He recorded 14 points, 12 rebounds and three blocks in a 74–65 win over VCU at the Atlantic 10 tournament final. He was named tournament most outstanding player.[9] As a junior, Osunniyi averaged 10.7 points, 9.4 rebounds, and 2.9 blocks per game.[10] He was again named to the Second Team All-Atlantic 10 as a senior as well as Defensive Player of the Year.[11]
^LeConey, Bill. "Osunniyi’s triple-double powers Mainland past Absegami", The Press of Atlantic City, December 21, 2016. Accessed March 17, 2021. "The Absegami High School boys basketball team found out just how tough it is to play against Mainland Regional’s 6-foot-8 center Osun Osunniyi on Wednesday night.... I’m just a rim protector,' said Osunniyi, a 6-foot-8 Somers Point resident."