Professor Hiroshi Ochanomizu, also known as Dr. O'Shay in the English dub of Little Astro Boy, is a central figure in that series as the primary mentor and guardian of the young robot hero. He is a respected scientist who holds a position of authority, often associated with the Ministry of Science, and is deeply passionate about the peaceful coexistence of humans and robots. His physical appearance is distinctive, characterized by a short, portly stature, a prominent and large nose, and receding curly white hair that frames the sides of his head. In flashbacks within Little Astro Boy that depict his younger years, he is shown working on a rocket, indicating a lifelong dedication to science and engineering.

In terms of personality, Dr. Ochanomizu is fundamentally kind, caring, and intellectually brilliant. He is a fervent believer in robot rights and justice, using his influence to advocate for these causes. Despite his high status as a scientist and leader, he remains a humble individual. He is known to be tremendously emotional and passionate about his work and his beliefs. However, he is also portrayed as being easily excitable and having a notoriously short temper, often resorting to loud shouting or becoming flustered when faced with foolishness or disagreement. This can manifest as a comedic tendency to sneeze repeatedly when particularly angry. Despite this flaw, his cunning intellect and deep sense of responsibility make him a formidable advocate for what is right.

Dr. Ochanomizu's primary motivation is to foster a world where humans and robots can live together in harmony. He believes that robots possess consciousness and emotion, and he is dedicated to protecting their rights. This philosophy directly drives his actions, particularly his decision to rescue and care for Astro. He serves as a father figure and legal guardian to the young robot, providing him with a home, guidance, and a moral compass. His role in the story is that of a mentor, scientific advisor, and protector. He often provides Astro with crucial information, technical support, and the wisdom needed to navigate complex moral and social challenges. In Little Astro Boy, he lives in a teapot-shaped house and guides the five-year-old Astro as he learns about the world.

His most significant relationship is with Astro, whom he treats as a beloved son. He is also connected to Dr. Umataro Tenma, Astro's original creator. While their history is complex and often strained due to Tenma's abandonment of Astro, Ochanomizu remains a figure who seeks to reform his former friend and colleague, never hesitating to call out his reckless behavior. In many continuities, including flashbacks in other series, they were once close friends and lab partners. Ochanomizu is also responsible for creating a robotic family for Astro, including a sister named Uran, to provide him with a sense of normalcy and emotional support.

Throughout the broader history of the character, Ochanomizu shows development by consistently learning to balance his scientific curiosity with his deep emotional connection to robots like Astro. He evolves from a brilliant but sometimes short-tempered scientist into a wise and patient leader. His notable abilities include his vast scientific knowledge, particularly in robotics and energy systems. He is a master engineer capable of creating advanced robots and is a skilled strategist and advocate, able to navigate political and legal systems to protect robot rights. He often utilizes his influence as the Minister of Science to enact change, such as promoting a Robot Bill of Rights.