Daijirō Ōhara serves as the section chief of the Kameari Park Police Box in the Katsushika area, making him the direct supervisor of the patrol officers there, most notably the troublemaking Kankichi Ryotsu. He is commonly addressed by his subordinates and colleagues as Buchō, meaning Division Head. Physically, Ōhara is a man of short and stocky build, with thick eyebrows and a short, square-shaped mustache that has become one of his most distinctive features. He almost always wears the complete standard police uniform, including the blue coat, matching trousers, police hat, and black tie, and carries a brown satchel when commuting to work. His commute is a lengthy one, taking two and a half hours each way by bus and train from his home in a fictional rural area of Chiba Prefecture, demonstrating his strong commitment to his job.

In terms of personality, Ōhara is a strict, serious, and diligent officer who takes great pride in maintaining the reputation and order of his police box. He embodies the values of a traditional, hardworking Japanese man from an older generation and can be quite conservative, often struggling to adapt to modern technology and new fads, which can sometimes lead to comical mishaps. Despite his often stern and short-tempered exterior, which is frequently on display when dealing with his subordinates' antics, he is fundamentally a modest, considerate, and caring individual. His most famous catchphrase, shouted in frustration, is Baka mon!, meaning "you idiot!," almost always directed at Ryotsu.

His primary motivation in the story is to ensure the smooth and proper functioning of his police box, a task made incredibly difficult by the laziness, disobedience, and get-rich-quick schemes of his most notable subordinate, Ryotsu. He is driven by a sense of duty to his superiors, like the chief of police, and a desire to uphold the dignity of his position and the police force as a whole. However, beneath his exasperation lies a deep, paternal care for the young officers under his command, particularly Ryotsu. Because he has no son of his own, he treats Ryotsu like a surrogate son, expressing his concern through tough discipline rather than open affection, though moments of genuine pride and empathy do surface.

Ōhara's role in the story is central as the anchor of authority at the police box. He is the beleaguered middle manager constantly trying to control the chaos caused by Ryotsu, often having to answer for his subordinate's behavior to higher-ranking officials. Despite their frequent conflicts, his relationship with Ryotsu is the most defining of the series and is rooted in their past. Decades before the main story, Ōhara arrested a drunken and disorderly Ryotsu, an encounter that reportedly inspired Ryotsu to join the police force. Ōhara also once gifted Ryotsu the comfortable sandals he still wears after noticing the young officer's feet were hurting from regulation shoes, highlighting his quiet thoughtfulness. Beyond Ryotsu, Ōhara has positive relationships with his other subordinates, trusting the wealthy and reliable Keiichi Nakagawa and the dedicated Reiko Akimoto, whom he often addresses with the warm suffix "-kun". At home, he finds stability with his wife, Yoshiko, who offers him crucial advice. He shares a warm bond with his daughter, Hiromi, having taught her the traditional tea ceremony. His protectiveness initially led him to fiercely oppose her marriage, but he eventually accepted it and later became a loving grandfather to her two children.

Throughout the long-running series, Ōhara shows a degree of development. While his core personality remains constant, he gradually adapts to the changing times. Although initially clumsy with the internet, video games, and mobile phones, he eventually learns to handle them as skillfully as the younger generation, showcasing an ability to master new hobbies and lifestyles quickly. He also ages visibly over the course of the story, with his granddaughter growing from a baby to a junior high school student, a progression acknowledged within the narrative.

Notable abilities and interests include proficiency in a wide range of traditional Japanese arts and disciplines. He practices kendo and judo, making him a formidable physical presence, and his skill in shogi (Japanese chess) is said to be among the best in the entire Katsushika Police department. His hobbies are numerous and include bonsai cultivation, calligraphy, the tea ceremony (sado), fishing, keeping koi fish, and watching period dramas. His most unusual and less admirable talent is his poetry recitation, which is notoriously awful. When he chants his poems, the result is described as being so unpleasant that it has been shown to frighten listeners, disturb the entire town, and even cause windows to shatter and buildings to quake.