is a Japanese tokusatsu series that serves as the 21st installment in the Kamen Rider franchise and the 12th entry in the Heisei era.
Main characters Eiji Hino is a politician's son who grew up in a lavish environment until his life was changed when he attempted to use his fortune to save a small African village, only to accidentally fund a civil war that killed a young girl he befriended. As a result, he renounced his lifestyle and became a wandering vagabond willing to help those in need. Returning to Japan to fund his travels, Eiji encounters the Greeed Ankh, who gives him the means to transform into Kamen Rider OOO and convinces him to fight Ankh's fellow Greeed in exchange for Cell Medals. Eiji agrees on the condition that he be allowed to use his Rider powers to help others and later allies himself with the Kougami Foundation amidst his battles with the Greeed.
After falling victim to Kiyoto Maki and the purple Core Medals due to his lack of selfish desires, Eiji slowly becomes a Greeed himself until the Kougami Foundation's president, Kousei Kougami, tells him that the only way to counteract the purple Core Medals is to acquire a desire of his own. Recalling his desire to gain the power to help others, Eiji eventually purges the purple Core Medals from his body, but loses his Rider powers and Ankh after the latter gives his life to help Eiji defeat Maki. Following the battle, Eiji realizes the power he sought comes from those he has bonds with and resumes his travels in the hopes of finding a way to resurrect Ankh.
During the events of the crossover film Kamen Rider × Kamen Rider Fourze & OOO: Movie War Mega Max, Eiji works with the Kougami Foundation as part of his efforts to resurrect Ankh. Eiji later receives help from a revived, future version of Ankh and regains his Rider powers while fighting Kamen Rider Poseidon. In the crossover film Kamen Rider Heisei Generations Final: Build & Ex-Aid with Legend Rider, Eiji creates a temporary simulacrum of Ankh via a replica body created by Kaisei Mogami. During the events of the 10th anniversary V-Cinema Kamen Rider OOO 10th: Core Medal of Resurrection, Eiji sacrifices himself to save a little girl from the First OOO, resurrecting Ankh in the process.
Utilizing three Core Medals in conjunction with the belt and the device, Eiji can transform into Kamen Rider OOO. While transformed, he wields the sword. His personal vehicle is the motorcycle, which can switch between its vending machine-like and its Honda Shadow-like . With the Core Medals, Eiji can either assume varying forms via a specific color-coded set or mix and match Core Medals of differing colors to assume varying . In both cases, he combines different animal powers in combat. Combos of a single color initially quickly drain Eiji's stamina due to their increased power, though he gradually acclimates to them over time. : Eiji's red/yellow/green-colored default form accessed from the , , and Medals that grants superhuman athleticism. In any form using the Tora Medal, he is equipped with the twin forearm-mounted aerokinetic . : An enhanced version of Tatoba Combo accessed from the First OOO's versions of the Taka, Tora, and Batta Medals that increases the capabilities of the original form. While assuming this form, Eiji is referred to as the . : The evolved form of Tatoba Combo accessed from the , , and Medals, which were created in the future, that grants chronokinesis. In this form, Eiji is equipped with an upgraded version of the Tora Claws called the . This form first appears in the crossover film Kamen Rider × Kamen Rider Fourze & OOO: Movie War Mega Max. : A green-colored auxiliary form accessed from Uva's , , and Batta Medals that grants the use of the self-duplicating ability. In any form using the Kamakiri Medal, Eiji dual wields the twin . The Kamakiri Medal first appears in the film Kamen Rider W Forever A to Z The Gaia Memories of Fate. : A yellow-colored auxiliary form accessed from Kazari's , Tora, and Medals that grants the use of the thermokinetic ability. This form is also required to tame the Ride Vendor's feral upgrade, the , which siphons the Core Medals' excess energy so that Eiji can maintain his transformation without overexerting himself. : A gray-colored auxiliary form accessed from Gamel's , , and Medals that grants gyrokinesis. In any form using the Gorilla Medal, Eiji is equipped with a pair of self-propelling gauntlets. : Eiji's red-colored super form accessed from Ankh's Taka, , and Medals that grants a pair of bird wings and the ability to fire feather-like energy projectiles. In any form using the Kujaku Medal, he is equipped with the pyrokinetic buckler. However, because of its increased power compared to the other single-colored Combos, using this form exhausts Eiji faster. During the final battle, Eiji utilizes the Taka Medal housing Ankh's consciousness to assume an enhanced version of this form where he gains the use of a familiar based on his Greeed partner for combat assistance. This form first appears in the non-canonical crossover film Kamen Rider × Kamen Rider OOO & W Featuring Skull: Movie War Core. : The evolved form of Tajadol Combo accessed from the , , and Medals, which were created from Eiji and Ankh's bond, that equips Eiji with an upgraded version of the Taja Spinner called the . This form appears exclusively in the V-Cinema anniversary film Kamen Rider OOO 10th: Core Medal of Resurrection where it is used Ankh while possessing Eiji. : A blue-colored auxiliary form accessed from Mezool's , , and Medals that grants self-liquefaction capabilities. In any form using the Denki Unagi Medal, Eiji dual wields the twin electrokinetic . : Eiji's purple-colored final form accessed from the , , and Medals that grants cryokinesis and the ability to damage non-purple Core Medals. In this form, he wields the , which can switch between and . He later gains the ability to wield the Medagabryu in his other Combos as he slowly becomes a Greeed, though only Putotyra Combo can control its Bazooka Mode. Due to the unique nature of the purple Core Medals, Eiji is unable to mix and match them with other Core Medals. Additionally, prolonged use of this form puts him at risk of succumbing to a berserker state and hastens his transformation into a Greeed. : A red/gold-colored special form accessed from the Taka, , and Medals that grants ergokinesis. This form appears exclusively in the anniversary film OOO, Den-O, All Riders: Let's Go Kamen Riders. : An orange-colored special form accessed from the , , and Medals that grants the use of the self-healing ability. In this form, Eiji wields the pungi, which allows him to summon a giant hooded cobra from the ponytail for combat assistance. This form first appears in the film Kamen Rider OOO Wonderful: The Shogun and the 21 Core Medals.
Eiji Hino is portrayed by .
Ankh is a dishonest and foulmouthed bird-themed Greeed who believes he has the right to take whatever he wants, sees people for their flaws, and is at odds with the other Greeed due to being a wild card. As revealed in the tie-in novel Novel: Kamen Rider OOO, after the First OOO removed his tenth Core Medal to finalize his creation 800 years before the events of the series, Ankh met a blind girl and created a Yummy to restore her sight out of empathy due to his own dulled senses. However, Ankh regretted his decision after the girl died of fright upon learning that her sight came from people that the Yummy murdered for their eyes. Honoring the girl's last request to protect people, Ankh sided with the First OOO to battle the other Greeed until the First OOO betrayed him to steal his Core Medals. After the First OOO was overwhelmed by the Core Medals' power and sealed the other Greeed, Ankh's right forearm and the Taka Medal housing his consciousness were severed from his body, which was sealed separately. As a result, Ankh lost most of his power.
When the Greeed are unsealed in the present, Ankh finds he cannot reform his missing body and steals the other Greeed's Core Medals to handicap them, only to accidentally lose his remaining Taka Medal to Eiji. Ankh intends to regain his other lost Core Medals and power by enlisting Eiji as the new Kamen Rider OOO, but the latter proves difficult to manipulate, so Ankh reluctantly cooperates with him to save lives on the condition that they gather Cell Medals so Ankh can maintain his form. Along the way, Ankh possesses the body of half-dead Detective Shingo Izumi to blend in with humans, which allows him to fully experience human sensations, but causes him to grow envious of humans. However, Ankh slowly finds himself unable to harm Eiji and the detective's sister Hina without a substantial reason.
As he regains some of his Core Medals, Ankh discovers what happened to the rest of his body after it develops a mind of its own and turns into Ankh (Lost). They subsequently compete to reabsorb each other under the belief that the weaker one will disappear forever. Ankh (Lost) initially proves victorious, but Eiji uses the purple Core Medals' power to kill him in retaliation for Ankh's apparent death, shattering a full set of his Core Medals in the process. Despite being able to reconstitute himself, Ankh finds he lost some of his power due to the Core Medals he lost and repossesses Shingo before seemingly joining Maki and the other Greeed in the hopes of secretly manipulating Eiji into destroying the other Greeed so he can use all of their Core Medals to fully fuse himself with Shingo and become a true conscious being.
While carrying out his scheme, Ankh unconditionally gives the other Greeed their Core Medals back as a result of his being spoiled by his experiences with human senses and disgusted with his fellow Greeed. However, he finds himself unable to kill Eiji due to their history together and his newfound capacity for feeling deeper emotions leads him to betray Maki, who responds by damaging the Taka Medal housing Ankh's consciousness. In spite of this, Ankh accepts his death and gives Eiji his remaining three Core Medals to counteract the latter's purple Core Medals long enough for Eiji to defeat Maki. After aiding him in the fight, Ankh's spirit appears before Eiji to express gratitude to him and the others for allowing him to live and die as a human, before giving him and Hina his broken Taka Medal.
During the events of the crossover film Kamen Rider × Kamen Rider Fourze & OOO: Movie War Mega Max, a revived future version of Ankh travels back in time to help Eiji defeat Kamen Rider Poseidon. During the events of the V-Cinema Kamen Rider OOO 10th: Core Medal of Resurrection, Eiji sacrifices himself to fully resurrect Ankh.
In his true form, known as , Ankh possesses pyrokinesis and flight capabilities. His bird-themed Yummy attack people to extract an attribute and feed it to their host, similarly to how mother birds feed their chicks. While possessing Shingo, Ankh uses his host's iPhone 4 and an iPad to gather information on the current era as well as keep track of who owns which Core Medals before receiving the case from Hina to store Core Medals in.
Ankh is portrayed by .
Hina Izumi is an aspiring fashion designer who works at the Cous Coussier restaurant and possesses superhuman strength, which she expresses embarrassment towards. Following the Greeed's awakening, her brother Shingo's death, and Ankh possessing him, Eiji initially tries to hide the truth from her before eventually telling her and vowing to save him. Realizing Eiji's reasons for looking after Ankh and Shingo, Hina convinces Eiji to work at Cous Coussier so she can care for her brother's body and use her strength to keep Ankh in line. She also comes to realize that she relies on Eiji's desire to help to an unhealthy degree, takes steps to ensure he does not blindly sacrifice himself for other people's sake, and cares for Ankh as if he were her brother.
Hina Izumi is portrayed by .