Miss Ōishi is the homeroom teacher of Totto-chan’s class at Tomoe Gakuen, the unconventional elementary school that becomes the center of the story. As a young and relatively inexperienced educator, she initially adheres to more traditional teaching methods, but she gradually adapts to the school’s unique philosophy under the guidance of Principal Sosaku Kobayashi. Her personality is earnest and well-meaning, though she occasionally makes mistakes that reveal her own learning process. In one notable instance, she comments to a student named Takashi that he might have a tail, a remark that deeply hurts the boy because of his small stature. Principal Kobayashi calls her into his office and firmly explains why such words are harmful. Miss Ōishi breaks down in tears, fully acknowledges her error, and sincerely apologizes, demonstrating her capacity for self-reflection and growth. This incident marks a turning point in her development as a teacher, helping her become more sensitive and attuned to the emotional needs of her students. Her role in the story is to represent the more conventional side of education that gradually gives way to the child-centered, respectful approach embodied by the principal. She oversees daily lessons, encourages students to pursue their own interests during free‑study periods, and supports them in activities such as outdoor walks and physical education. Her key relationships include the one with Principal Kobayashi, whom she respects and learns from, and her bond with Totto‑chan and the other children, whom she comes to see as individuals rather than problems. Miss Ōishi does not possess any special abilities beyond her teaching vocation, but her notable trait is her willingness to admit fault and change, illustrating the theme that even adults can grow through empathy and humility. Her presence reinforces the idea that a supportive school environment requires not only a visionary leader but also dedicated teachers who are open to improvement.