On April 22, 2012, Osawa fought in the quarterfinals of the Krush Super Featherweight tournament, being scheduled to fight Yuya Yamato.
The fight would end in a disqualification win for Osawa, as Urabe landed an illegal punch as the referee was separating the two from a clinch entanglement.
[9] Following an unbeaten streak of four fights, Osawa was scheduled to make his K-1 debut on July 4, 2015, in a rematch with Kotaro Shimano.
[11] Following a 3–2 run, which ended in a decision loss to Masanobu Goshu,[12] Osawa moved up in weight to lightweight.
He won a majority decision against Wei Liu in the quarterfinals, and beat Kyoshiro by a second-round TKO in the semifinals.
[21] He was scheduled to fight a rematch with Yuto Shinohara in December 2019, but had to withdraw after suffering an injury in training.
[23] He was then scheduled to fight Shuji Kawarada at K-1:Winter's Crucial Bout,[24] and lost by split decision.
[25] Osawa snapped the two-fight losing streak with an extra round unanimous decision against Shinichiro Kawasaki at Krush 123.
[32] Osawa faced the 2020 True4U bantamweight tournament winner Dansiam Ayothayafightgym at K-1 World GP 2022 in Fukuoka on August 11, 2022.
[36] The result was changed into a no contest on January 29, 2023, as Osawa scored the sole knockdown of the fight off of an illegal kick catch.