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.

[7] In his junior season, Osunniyi was a Second Team All-Atlantic 10 selection and Atlantic 10 Defensive Player of the Year.

His name, Osun Bowale, translates to "angel of love and fertility has returned home.