Osamu Nagayama is a recurring supporting character in Chibi Maruko-chan, where he appears as a classmate of the main character in the third grade class at Irie Elementary School. He is a boy with neatly cut brown hair and glasses, and he is widely regarded within the class as one of the most intelligent and well-informed students.

In terms of background, Osamu is a nine-year-old third grader who lives with his family, including a younger sister named Koharu. Koharu is somewhat frail and often unwell, and Osamu takes his role as her older brother very seriously, looking after her with great care and devotion. His birthday is May 2 and his blood type is B.

Personality-wise, Osamu is kind, calm, and polite. When classmates approach him with questions, he answers them clearly, patiently, and courteously, which makes him approachable despite his strong academic reputation. His knowledge is not limited to schoolwork; he is also curious about the wider world, with a particular strength in nature and living things. He has a broad-minded and fair disposition, as shown when a classmate was being blamed by others and he continued to treat her normally rather than joining in. His hobbies include observing the stars and drawing, and his dream for the future is to become a manga artist. His favorite foods are spaghetti and croquette.

Osamu's main motivations in the story are his desire to be helpful to others and his personal aspiration to become a cartoonist. He genuinely enjoys sharing what he knows, and he often assists classmates with their studies and research. On one notable occasion, he helped Maruko prepare for a test, contributing to her achieving a perfect score. He also takes pride in caring for his sister, and his gentleness toward her reflects the same dependable nature he shows at school.

In the story, Osamu functions as a reliable supporting figure within the class. He is not a central leader like some of the other boys, but he is well liked and respected, and classmates regularly turn to him when they need information, explanations, or help with schoolwork. His relationship with Maruko is friendly and cooperative; she asks him for help and he responds kindly. His relationship with his sister Koharu is the most significant family tie shown, and it highlights his nurturing side.

Osamu does not undergo dramatic development over the course of the series; he is a consistent presence whose good nature and intelligence remain stable. His notable abilities include strong academic performance, a wide range of general knowledge especially about the natural world, the ability to explain things clearly and patiently, and a talent for drawing that aligns with his ambition to become a manga artist. His interest in astronomy also shows a thoughtful, observant side to his character.