Kōji Ōishi is a Japanese manga artist born on July 14, 1982, in Kumamoto Prefecture. He made his professional debut in 2004 with the one-shot manga "Kyōdai Jingi," which was published in Weekly Shōnen Jump as a replacement for another series. He first gained attention in 2005 as a participant in the magazine's Golden Future Cup contest with the one-shot "Baka in the City!!," a prototype for his later work.

Ōishi's first serialized manga, "Maison de Penguin," ran in Weekly Shōnen Jump from 2006 to 2007. This work was notable for being an unusual presence in the magazine, as it employed a four-panel comic strip format and a sketch-like art style to deliver its episodic comedy. Following this, he created a short series titled "Project SQ," a report-style manga that depicted the behind-the-scenes creation of the new magazine Jump SQ.

He is best known for the long-running comedy series "Inumarudashi," which was serialized in Weekly Shōnen Jump from 2008 to 2012. His other major works include "Tomatoipu no Lycopene," which began as a one-shot before becoming a serialized series from 2017 to 2025, and "Maid in Cat". In the field of anime, Ōishi served as the original creator and character designer for the web anime "Kiyoneko," which began streaming in December 2022. This series is based on a character created by the game commentator Kiyo.

The artistic identity of Kōji Ōishi is defined by gag comedy. His style frequently incorporates surreal and self-deprecating humor, along with risqué jokes, current event parodies, and references to other works. He often creates a comedic contrast by using cute, fancy character designs for animals and children within his absurd storylines. One of his most notable industry distinctions was a cameo appearance in the manga "Bakuman.," where he appeared as a fictionalized version of himself. In the television anime adaptation of "Bakuman.," he further crossed the boundary between mediums by voicing this animated version of himself.