An older male teacher at Futatsubashi Elementary School. The only one of the four main teachers seen in the series with children, specifically a daughter, Oyajima has an easygoing outlook on life and teaching and frequently dispenses advice and words of wisdom to his younger, more easily excitable work colleagues. Rarely flustered, he is the senior of Sae Sharai and has a visible soft spot for her, often offering insight to Hōin concerning their mutual colleague's behavior, background, and mindset in his usual, laidback style without coming off as defensive or biased. His good working relationship with Shirai also allows him to present her with alternative points of view or make thought-provoking observational comments in a gentle, nonconfrontational manner without either one of them getting worked up in the process.