Oishi made his debut in 1993, in Shooto, when he faced Yoshiaki Murai, winning a unanimous decision.
He would accumulate a 7-5-3- record, but earned a title shot off of a two fight win streak over Masaki Nishizawa and Yoshinobu Ota.
[4] He defended the title versus Hisao Ikeda,[5] before losing it to Ryota Matsune through a unanimous decision.
He lost to Hideo Tokoro in the opening of the ZST Grand Prix,[7] which was considered an upset at the time.
Moving to Cage Force, he earned a title shot by beating Paul McVeigh and Tetsu Suzuki.