Koshi Osaki is a Japanese defender from Osaka Prefecture, born on 30 June 1998. He played for Tokai University Gyosei High School and Osaka University of Health and Sport Sciences before joining Mito HollyHock for the 2021 season. Osaki made his J.League debut against Zweigen Kanazawa in May 2021 and scored his first league goal against SC Sagamihara the following month. He has remained with Mito as a left-footed defensive option.