Kōji Ōji is the original creator behind the light novel series High Speed!, which served as the foundation for the popular Free! anime franchise. Ōji wrote the two-volume light novel series, which was published by Kyoto Animation under their KA Esuma Bunko imprint. The first volume was released on July 8, 2013, and the second volume followed on July 2, 2014. Both volumes feature illustrations by Futoshi Nishiya.

Ōji's work first gained recognition when High Speed! received an honorable mention in the light novel category at the second Kyoto Animation Award contest in 2011. This award led to the novel being published and subsequently adapted into the anime series Free!. While the Free! anime series expanded upon the concepts and characters from High Speed!, Ōji is credited specifically as the author of the original source material.

The first volume of High Speed! follows Haruka Nanase, Makoto Tachibana, Nagisa Hazuki, and Rin Matsuoka during their elementary school years as members of a swimming club. The second volume continues the story as Haruka and Makoto enter junior high school and join the swim team alongside new characters Asahi Shiina and Ikuya Kirishima. This second volume was loosely adapted into the 2015 animated film High Speed! Free! Starting Days, for which Ōji received credit for the original story. The film was directed by Yasuhiro Takemoto, with a screenplay by Masahiro Yokotani and Maiko Nishioka.

Ōji's significance in the anime industry stems from his work serving as the narrative origin for one of Kyoto Animation's most commercially successful franchises. The Free! series expanded from his light novels into multiple television seasons, theatrical films, and various other media adaptations. A manga adaptation of High Speed! with artwork by Shiori Teshirogi was also released in two volumes on August 14, 2015, to promote the animated film.