Tomohito Ōsaki is a Japanese playwright, director, and novelist known for his work in both theater and the adaptation of popular manga series into prose and other media. Born in Hiroshima Prefecture in 1976, he graduated from the Faculty of Letters at Doshisha University, where he was a member of a comedy research circle. His writing career began in 1999 when his work Running Op was selected for an award in the Jump Novel Contest under the pen name Tomohiro Oki.

Ōsaki has a significant background in theater, particularly with Yoshimoto Kogyo, where he has written and directed numerous stage productions for Yoshimoto Shinkigeki at Namba Grand Kagetsu. Many of these works were performed with comedian Kazutoyo Koyabu as the lead.

As a novelist, Ōsaki is widely recognized for his novelizations of popular Shueisha manga properties. A notable example is his work on the Gintama franchise. He is the author of the light novel series 3-Nen Z-Gumi Ginpachi-sensei, which reimagines the characters of Hideaki Sorachi's Gintama in a high school setting. The first volume was published by Shueisha under their Jump J-Books imprint in February 2006, with new volumes continuing to be released as recently as October 2025. This light novel series later served as the basis for both a manga adaptation, for which Ōsaki served as a supervisor, and an anime television series that aired in late 2025.

Beyond Gintama, his body of novelization work includes titles for other major shōnen manga. He wrote novel adaptations for BLACK CAT by Yabuki Kentarō and Nura: Rise of the Yokai Clan by Hiroshi Shiibashi. He is also the author of NARUTO: Sakura's Story—Love Riding on the Spring Breeze, part of the Naruto novel series.

Ōsaki is also the original creator behind the short story collection One Piece novel Mugiwara Stories, based on the One Piece franchise by Eiichiro Oda. This collection of stories, which focuses on everyday characters whose lives are touched by the main cast, was first serialized in One Piece Magazine before being compiled into a single volume in November 2017. This work was adapted into the acclaimed anime special One Piece Fan Letter, which aired in October 2024 to celebrate the anime's 25th anniversary.

In his original works, Ōsaki often engages with established fictional universes, crafting new narratives and spin-offs that expand upon their worlds. His background in comedy theater informs his writing, particularly evident in the humorous and satirical tone of the Ginpachi-sensei series. His career demonstrates a significant role within the manga and anime industry as a skilled adapter and expander of popular franchises, translating their characters and worlds into new formats and stories.