[5] He alternated wins and losses over his next six fights, defeating Yuzo Suzuki, Masafumi Kurasaki and Hikaru Fujihashi, but losing to Yuta Otaki, Hirotaka Asahisa and Kaito Ozawa.
[10] The winning streak was snapped by Taio Asahisa, who won a unanimous decision during Krush 82.
He first defeated Kyohei Hayashi by a first-round KO,[18] and won the rematch with Ryusei Ashizawa by unanimous decision.
[26] Oiwa was scheduled to fight the former K-1 Featherweight champion Yuki Egawa at K-1 World GP 2021 in Fukuoka.
[32] Oiwa wax expected to face Kaito Ozawa at K-1 World GP 2023: K'Festa 6 on March 12, 2023.
[35] Oiwa faced Tomoya Yokoyama at K-1 World GP 2023 in Yokohama on June 3, 2023.
[37] Oiwa faced Yuto Shinohara, in his first fight at the lightweight (-62.5 kg) limit, at K-1 ReBIRTH 2 on December 9, 2023.