Onizuka had already been accepted to Kinki University, but decided to turn pro after being battered in a sparring session with Hiroki Ioka, who had just become the WBC Minimumweight champion.
He fought Mexican fighter Armando Castro three months later for his second defense, and won an indisputable victory.
Onizuka's third defense came on May 21, 1993, against Korean fighter Jae-Shin Lim, who had previously fought with the alias, "Kotaro Hayashi."
After his controversial fight, Onizuka fought Thanomsak Sithbaobay for the second time, and made his fourth defense by another close 12-round decision.
The referee finally stopped the fight in the 9th round, and Onizuka lost his title after two years of controversial defences.
Fans were reminded of Onizuka's controversial wins when Koki Kameda won a hometown decision to become the WBA Light Flyweight champion in 2006.