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Official Site: Bandai

is the primary and eponym protagonist of the 2010-2011 Kamen Rider Series Kamen Rider OOO. The name "OOO" is pronounced similarly to both "O's", referring to the three O's in the television series' title which represent the character's transformation items, and . The three O's also represent the infinity symbol with an additional circle, stating that Kamen Rider OOO is Over (infinity).

Kamen Rider OOO makes his debut appearance in Kamen Rider W Forever: A to Z/The Gaia Memories of Fate where he faces off against and defeats the Luna Dopant. An alternate spelling of the character's name is "Kamen Rider Ooz", which comes from Hyper Hobby magazine's September 2010 issue.

is the son of a politician who travels the world with his grandfather, thus growing up in a lavish environment. While in Africa a year prior to the beginning of the series, Eiji attempted to use his fortune to help out a small village. However, his money was used to fund a civil war that cost the life of a little girl Eiji befriended, whose death he was unable stop, forced to looked on helplessly. Though the Hino family used his sob story to get the people's support, the ordeal perculiarly caused Eiji to lose all selfish desires, resulting him being a vagabond with no permanent job, no home, almost penniless and no life. He became willing to help those in need, often going so far as to risk his life for complete strangers. Eiji's rationale for helping people is if someone reaches their hand out to him and he does not reach back, he will regret it for the rest of his life.

Returning to Japan to allegedly fund more of his world travels, Eiji's latest part-time job at the Kougami Art Museum ends due to the destruction of the museum around him while in a drugged slumber, taking a strange red coin he found as his supposed pay. This causes a disembodied arm to begin to follow him around, demanding the red coin from him which is actually a part of his being. However when attacked by a Yummy, the arm reveals himself as Ankh of the ancient beings known as the Greeed, and the red coin is one of his Core Medals. When Eiji risks his life to rescue him, Ankh gives him two more Core Medals and the OOO Driver as means to transform into Kamen Rider OOO. After destroying the Yummy, Eiji learns more about the Greeed and Medals from Ankh, who originally intends for Eiji to be a tool for gathering Cell Medals for him. However, after the Greeed take the half dead body of Shingo Izumi for his own, Eiji manages to force Ankh into promising him that he can transform to save lives, not just to gather Cell Medals. Feeling guilty for getting Shingo involved, Eiji remains by Ankh's side as the two are joined by Shingo's little sister Hina Izumi.

Kamen Rider OOO has not been a contender in any CBUB matches.