He returned to Japan to fund for more of his travels. His latest part-time job at the Kougami Art Museum ends due to the destruction of the museum around him, Eiji takes a strange red coin he found while off duty which he thinks is his pay. This causes a disembodied arm to begin to follow him around, until the arm announces that it is called Ankh and is one of the ancient beings known as the Greeed, and he needs Eiji's help to become King. When they are attacked by a monster called a Yummy, Eiji risks his life to rescue Ankh, who in return saves him and gives Eiji two more strange coins, calling them Medals, and a belt, enabling Eiji to transform into Kamen Rider OOO.
After destroying the Yummy and further encounters with the Greeed, people from the Kougami Foundation begin to approach Eiji, offering their assistance in the fight against the Greeed and Yummy, but their true motives are not clear. He also gets a job and housing at the Cous Cossuier after the owner Chiyoko finds out that he is a world traveler. In the process, joined by Akira Date, Eiji learns more of the Greeed's desire to become whole and the reason why Ankh is unable to assume his complete form. However, things changed when Eiji ended up becoming a host to half of a set of Purple Core Medals that react to his lack of selfish desires, the same presumed reason why he is able to use Kamen Rider OOO's powers without any sign of going berserk so far like the First OOO, with dangerous consequences. Soon after, Eiji learns from Kousei Kougami, the Foundation's owner, that the only way to counteract the Purple Core Medals is to acquire a desire of his own, something he is yet capable of.Later on, he turns into the dinosaur-like Eiji Greeed after absorbing two more purple Core Medals from the other Purple Greeed, Kiyoto Maki, in a bid to make him go beserk. However, it was by then Eiji remembers his desire is to have the power to help others and later agrees to Kougami's suggestion to absorb all the Cell Medals in the Kougami Foundation's possession to achieve that desire. However, it does not halt Eiji's Greedification during his his final battle when he uses all the Cell Medals in his body in a risky attack that fails to destroy Maki. But before Eiji fully becomes a Greeed, Ankh gives Eiji his last three Core Medals, disappearing from Shingo and allowing Eiji to transform to defeat Maki, resulting in the destruction of the Core Medals along with Ankh's. As a now-normal Eiji falls back to the Earth, Ankh appears to him as his disembodied arm, saying his farewells, before disappearing into half of the broken Taka Medal that once held his consciousness. Saved by Kamen Rider Birth, receiving the other half of Ankh's Core Medal that Hina found, Eiji leaves Japan and resumes his journey around the world while find the way to fix the broken Taka Medal with a spectral Ankh arm following him.
Eiji would later return in Kamen Rider × Kamen Rider Fourze & OOO: Movie War Mega Max where he returns to Japan to battle Kamen Rider Poseidon from the future who absorbed the Core Medals from the end of the series who wnated to end the Kamen Riders of the modern era, strangely enough, Eiji wore Ankh's gaunlet and forcibly removed the Taka, Tora and Batta Medals within Poseidon, revealing to be a diguise worn by Ankh to let Poseidon's guard down as the real Eiji Hino appears as hei and Ankh would go on to defeat Kamen Rider Poseidon by teaming up Kamen Rider Aqua with all the Core Medals back in the modern era except for the Purple Core Medals, Foundation X would appear to take the Core Medals of Poseidon for their scheme and Ankh mysteriously disappears into a warp hole that leads into the future, Ankh's Taka Medal is still broken as Eiji reveals to Hina, indicating sometime in the future, Ankh's Medal would be repaired if they kept up with what they are doing in the present.
Eiji would meet up with his successor, Kamen Rider Fourze and teams up with him and his predesscor, Kamen Rider Double/W to take on Foundation X to end their schemes, he is also given three Core Medals from the future by Michal Minato which are the Super Taka, Super Tora and Super Batta Medals, allowing him to fight on even grounds against the Galaxy King and together with Fourze Rocket States defeating him.
Eiji will also appear in Kamen Rider × Super Sentai: Super Hero Taisen, a crossover between the two shows, Super Sentai and Kamen Rider as their respective enemies team up and in turn, the Kamen Riders must team up with Super Sentai, Eiji is seen using all of his Combos and the Purple Core Medals somehow returning, he also seen giving his approval to Kaizoku Sentai Gokaiger to use Ranger Keys based off GataKiriba, Latoratah, SaGoZo, ShaUTa, TaJaDol and PuToTyrannosarus respectively.
Despite often coming off as happy-go-lucky in his personality, Eiji is actually quite strong-willed and was easily able to pick up on the fact he could use Ankh's need for him and the OOO Driver to keep the Greeed in line. Another defining trait is Eiji will fight to protect anyone in need, even complete strangers and at the risk of his own life. This stemmed after his regret from not saving the little African girl he befriended during a civil war which his family fortune started. He keeps a collection of coins and almost always has pairs of underwear of different designs and colors which he at times even gives to others. Eiji possesses no selfish desires which made him ideal for being OOO but also became a weakness once the Purple Core Medals went into his body.
Eiji arrives in Sky City with all of the Core Medals apart from the Orange, Imagin and Shocker Medals and fights alongside the heroes, at first using mix-and-match Combos before being forced to Medal Combos, especially against the Dark Pieces, he would go on to travel with the others to battle the Unbreakable Dark as Kikyoto Maki provided him spare Medals he somehwo acquired and with all of the Medals, he was able to transform into GataKiriBa Combo and use it's Branch Shade ability to duplicate himself and the copies switched out the Green Medals for the others, allowing Eiji to use all of the Combos at the same time minus the TaMaShii Combo and with a combined effort with Ankh in his Greeed form and him in PuToTyrannosarus, effectively managing to bring her down to the ground for the final blows for the Protaganists.
Eiji returned to Japan and continued the fight against the Greeed and with Ankh, defeat Maki but at the cost of losing all the Core Medals and left with shattered halves of Ankh's Taka Core Medal that held his soul and so he went off to travel in the world to discover a means to bring his friend back.
Eiji arrives in Angel Grove in America as the invasion begun, witnessing Kazari fighting the Power Rangers as he never returned to Japan after the beach party and almost killed but as soon as Kazari's guard was down, Eiji punched through his back, wearing Ankh's gaunlet and forcibly removed a few Core Medals, including the Taka, Tora and Batta Medals and thrown them behind Kazari, revealing the true Eiji Hino as the latter was in fact, Ankh in disguise.
Eiji would join a Disgaea Tournament with fellow Kamen Riders Decade, Double, and Aqua respectively being ironcally named Team Kamen Riders with all but the Purple Core Medals in his possession except for the Kujaku, Unagi and Tako Medals.
After Eiji inserts three Core Medals into the OOO Driver, he scans them with the O-Scanner, and then holds the O-Scanner up to his chest, he transforms into Kamen Rider OOO. Like Kamen Rider Double before him, OOO has multiple combinations of forms he can access referred to as Combos, he may use mix-and-match Medals or two of the same colour with one different Medal, Eiji is also able to three a full set of Medals to create a Medal Combo, it does give him a powerful advantage but Medal Combos are taxing on Eiji's body, especially Ankh's and the Purple Combo's taxing effect starting earlier than the others, eventually, Eiji notes that his body is "getting used to the power", though he must still be cautious with excessive Combo use. Except for Tatoba and Tamashii Combos, the only combinations with "Combo" in their name are those of a single color. Combos are announced by the O Scanner with a short song made up of the syllables of a Combo's name, a quirk Ankh advises Eiji to ignore.
Kougami tells Eiji Hino that he is actually the second to be Kamen Rider OOO, with Ankh later stating that only Eiji can be the current Kamen Rider OOO as he was the one who undid the seal of the OOO Driver. Ankh later revealed that the first Kamen Rider OOO was a king in Europe 800 years ago. However, because of his desire for godhood, the power warped the first OOO's mind before he was overwhelmed by desire while using the powers of all the Core Medals at once, thus turning his body into a stone coffin which sealed the Greeed. But due to his lack of personal desire, Eiji is theorized by Kougami to be capable of making such a transition into becoming a living god.
Eiji's Greeedification also slowly changes his human body to that of a Greeed's as he slowly loses his five senses as the process continues, his vison being whited out, sense of taste disappearing, sounds are distorted, no smell and finally no sense of touch, however with it, Eiji is able to sense Greeed or Yummy as hsi eyes turn violet purple, he also able to exert a burst of dark violet wind from his body, he is even able to fiire balls of purple flame and summon forth his Greeed-like limb to deflect a projectile attack, as he is becoming a Greeed, he is able to absorb Cell Medals inside his body, reinforcing his body and his attacks as evidenced in Episode 48 when he exits the Kougami Foundation, the concrete floor crumbles under his steps and even dealing minor damage to Kiyoto maki who did sucessfully block, bleeding a few Cell Medals.
Eiji also has the power to completely transform into his Greeed form and from it, directly enter into PuToTyrannosarus without needing to insert the Medals into the Belt.
CombosEditTaToBa Combo is Kamen Rider OOO's main combination, using the powers of the Taka, Tora, and Batta Medals. As Tatoba Combo, Kamen Rider OOO can see far distances, use the Tora Claw gauntlets, and can jump and kick with great strength. This Combo's Scanning Charge finisher is a Rider Kick called the Tatoba Kick, where after jumping high up with the power of the Batta Legs, Kamen Rider OOO descends with a Rider Kick drop kick through the three rings that form, surrounding him with a different aura based on the Taka, Tora, and Batta Medals
- Eiji transforms into a purple-eyed Tatoba Combo due to Doctor Maki transferring one of his Core Medals into Eiji's body, turning him into a mindless monster to attack Ankh before transferring another to cause Eiji to transform directly into the Eiji Greeed.
- After absorbing a vast amount of Cell Medals, Eiji uses the King's Core Medals which Kougami gave him to transform into a more powerful version of Tatoba Combo, referred to as "the True OOO" by Kougami. In this state, Kamen Rider OOO's power has increased to the point of momentarily surpassing the Purple Core Medals, allowing him to dual wield the Medajaribur and the Medagabryu, as well as defeat a complete Greeed with the Tatoba Kick. Also, the projection of the three rings of Medals that manifest during OOO's usual transformations are replaced with floating projections of the Taka, Tora, and Batta Medals only, before forming the O Lung Circle image of Tatoba.
GataKiriBa Combo, also known as the "Strongest Combo" is Kamen Rider OOO's green combination, using the powers of Uva's Core Medals: Kuwagata, Kamakiri, and Batta. As Gatakiriba Combo, Kamen Rider OOO can create massive numbers of duplicates of himself called the Branch Shade, shoot lightning from the Kuwagata Horns, use the Kamakiri Swords, and jump high with the Batta Legs. However, Eiji claims it is extremely difficult to revert back to being just a single Kamen Rider OOO so that he can cancel the transformation. This Combo's Scanning Charge finisher is a Rider Kick called the Gatakiriba Kick, where Kamen Rider OOO and his many duplicates all jump into the air and converge on the opponent withflying sidekicks, reintegrating back to one after hitting the target.The duplicates are also capable of transforming into other Combos but they disappear after a Scanning Charge, leaving just the original OOO.
Latorartar Combo, also known as the "Scorching Combo", is Kamen Rider OOO's yellow combination, using the powers of Kazari's Core Medals: Lion, Tora, and Cheetah. As Latorartar Combo, Kamen Rider OOO can emit an infrared light called the Liodias that is intense enough to incinerate, use the Tora Claws, and use the Cheetah Legs to run at blurring speeds and perform a rapid barrage of kicks. One of his attacks in this Combo is the Revol Spin Kick. This Combo's Scanning Charge finisher is the Gush Cross, where Kamen Rider OOO projects three yellow rings in front of him before charging through them with the Cheetah Medal's speed, delivering a cross slash on the target with the Tora Claws infused with the Lion Medal's energy projection.Sagohzo Combo, also known as the "Gravity Combo", is Kamen Rider OOO's gray combination, using the powers of Gamel's Core Medals: Sai, Gorilla, and Zo. As Sagohzo Combo, Kamen Rider OOO gains superhuman strength and gravity manipulation from the medals, able to create powerful shock waves by banging his Gorilla Arms against his chest, execute strong headbutt attacks, use the Gori Bagōn, and cause minor earthquakes with the Zo Legs. Some of his attacks in this Combo are the Bagōn Pressure,, where the Gori Bagōn gauntlets are launched from his arms, and a powerful dropping attack called the Zō Stomp. This Combo's Scanning Charge finisher is the Sagohzo Impact, where Kamen Rider OOO jumps in the air and does a powerful stomp, which traps a Yummy in the ground, pulling it towards him so he can deliver a powerful double handed punch and headbutt at the same time. TaJaDol,, also known as the "Blazing Combo", is Kamen Rider OOO's red combination, using the powers of Ankh's Core Medals: Taka, Kujaku, and Condor, with the O Lung Circle on his chest forming the image of a phoenix. As Tajadol Combo, Kamen Rider OOO can see far distances, create energy peacock tail feathers as projectiles to throw at opponents, use the Kujaku Wings to fly, shoot streams of fire, and use the talon-like Raptor Edges of the Condor Legs. In addition, he is armed with the Taja Spinner, which can shoot fire disks. A Giga Scan can be done by replacing some of the Cell Medals in the Spinner with Core Medals, then scanning it with the O Scanner. This Combo's Scanning Charge finisher is a Rider Kick called the Prominence Drop, where after sprouting wings from the Kujaku Arms, Kamen Rider OOO flies into the air, flips around and performs a flying drop kick on the target with the Raptor Edges extending its talons as he ignites. As this combo is a Super Combo, it is more powerful than any other Combos. The downside is that Tajadol Combo is even more taxing on Eiji's body, with him feeling the effects of physical strain much sooner and much more intensely than with other Combos. Though Kamen Rider OOO first uses this Combo to fight the Lion-Kurage Yummy in episode 20, Tajadol Combo debuts during the events of 'Kamen Rider × Kamen Rider OOO & W Featuring Skull: Movie War Core to finish Kamen Rider Core alongside Kamen Rider Double CycloneJoker Gold Xtreme
- During the final battle, Ankh gives Eiji the last two Kujaku and Condor Medals as well as the last Taka Medal that contains Ankh's consciousness, that was shattering, in order to fight Maki in his Greeed form, manifested as during the usual projection of Medals during transformation as red with yellow and green accents rather than the usual pure red. When Eiji transforms into Tajadol Combo, Ankh's voice announces the name of the Medals instead of the usual O Scanner voice. Ankh's consciousness fights along side Kamen Rider OOO and performs a finshing attack along with the Giga Scan using the Purple Core Medals.
Tamashii Combo, also known as the "Soul Combo" and the "Monster Combo", is Kamen Rider OOO's special combination, that appears in the OOO, Den-O, All Riders: Let's Go Kamen Riders film, using the powers of Taka, Imagin, and Shocker Medals. As Tamashii Combo, Kamen Rider OOO can see far distances, use the Orenotsuno, and the talon-like attachments of the Shocker Legs. This Combo's Scanning Charge finisher is the Tamashii Bomber, where the Shocker symbol appears on an enemy before Kamen Rider OOO launches a ball of energy made from the Imagin Arms.Putotyra Combo, also known as the "Invincible Combo", is Kamen Rider OOO's purple combination using the powers of the purple Core Medals that forced themselves into Eiji's body: Ptera, Torikera, and Tyranno. Though it is the most powerful form that Kamen Rider OOO can assume, it originally turns Eiji into a mindless beast who attacks friend and foe alike as soon as the combo is activated. He eventually manages to exert his will over the purple Core Medals, but he still suffers from the physical strain resulting from Combo use, and is still at risk of "letting himself go" if he does not consciously keep control. As a result of the purple Medals residing in his body, Eiji is undergoing a process known as "Greeedification", gradually turning into a Greeed. The Putotyra Combo has a power that is capable of destroying or damaging Core Medals. After Eiji gains two more Purple Medals, he can access this from in the usual way or straight into Putotyra while in Greeed form. As Putotyra Combo, Kamen Rider OOO can freeze opponents with a cold wind, use the wing-like External Fins on the Ptera Head to fly, use the Wind Stingers that can extend and attack the enemy from far, and is armed with the Tail Divider that can deliver a powerful blow and the Legsteranail claws for powerful kicks. In addition, he is armed with the Medagabryu. A Putotyranno Hissatsu can be done by placing Cell Medals in the Axe Mode's slot, then closing the hatch. This Combo's Scanning Charge finisher is the Blasting Freezer, where Kamen Rider OOO restrains the target with the Wind Stingers, then uses the External Fins to freeze it, and finally shatters the frozen target with the Tail Divider. Unlike other Combos, the OOO Armor becomes white and cannot mix-and-match with other Medals, due to their Medals' nature as the antithesis of desire negating the power of the other Core Medals, and it is announced by the call "Putotyrannosaurus" rather than its name "Putotyra".
Burakawani Combo, also known as the "Reptile Combo" is Kamen Rider OOO's orange combination that only appears in the film Kamen Rider OOO Wonderful: The Shogun and the 21 Core Medals. It uses the powers of the three reptile-themed Core Medals: Cobra, Kame, and Wani, that were last known to be in the possession of Tokugawa Yoshimune. But when Eiji and company appear in Edo due to a time distortion caused by Gara, Eiji gains the Core Medals during his battle against the Nue Yummy. Burakawani Combo's body is covered in a substance called Soma Venom, which makes him able to heal instantly after taking any attack. As Burakawani Combo, Kamen Rider OOO's eyesight is doubled and gains infra-red vision, uses the turtle shell-like Goura Guardner, which can be put together to form an energy shield called the Gōra Shīeld, and the Saw Deadscythers on his legs project the image of crocodile heads shown "biting" the opponent when Kamen Rider OOO's kicks come in contact. In addition, he is armed with the Burahngi, which lets him summon a giant cobra from the top of his head to attack enemies. This Combo's Scanning Charge finisher is a Rider Kick called the Warning Ride, where Kamen Rider OOO projects three orange rings in front of him before dashing through them in a foot-first baseball slide, unleashing a powerful kick with the Saw Deadscythers biting the opponent.
Super Tatoba Combo is a more powerful version of Tatoba Combo, using the powers of the Super Taka, Super Tora, and Super Batta Medals. Super Tatoba Combo has the Super Taka Head, similar to the Taka Head Brave as Tajadol Combo, the longer and sharper Super Tora Arms, and the spiked Super Batta Legs. This Combo's Scanning Charge finisher is called the Super Tatoba Kick, a more powerful version of the Tatoba Kick, since these Core Medals come from the future they have the ability to speed up it's wearer to to what people perceive as stopping time when in fact theya re perhaps moving at more faster speeds to the naked eye, when facing the Super Galaxy KIng who used Poseidon's Core Medals and the SOLU Switch to harness cosmic energy, he could easily handle two Kamen Riders at once until Eiji uses the Super Taka, Tora and Batta Medals to even the odds by attacking ta the same speed as the Super Galaxy King
In addition to the above Combos, there are also Random Combos' consisting of any other combination of three Core Medals.
- Eiji simultaneously uses the TaToBa, GataKiriBa, LaToraTah, SaGoZo, TaJaDol, ShaUTa, PuToTyrannosarus and BraWaKani Combos against Unbreakable Dark is a reference to the movie, Kamen Rider OOO Wonderful: The Shogun and the 21 Core Medals where he uses all said Combos to defeat the rogue alchemist, Gara.
- The Themes for OOO's Medal Combos are in fact plays on the Combo's name liek for example TaJaDol can be a play for Time Judged All and sung by Shu Watanabe who plays the role of Eiji Hino, the Combo titles were also sold under the name Eiji Hino, he even performed a duet with Ryosuke Miura, Ankh's actor for Time Judged All, the theme behind OOO's Blazing Combo using Ankh's Core Medals and another duet with Akira Kushida who is the voice of the O-Scanner in Power to Tearer.
- In the World Arc, Eiji Hino will be meeting Captain Marvelous of Kaizoku Sentai Gokaiger along with Kamen Rider Decade and it is most likely Marvelous, Doc, Luka and Ahim will have access to the TaJaDol, GataKiriBa, LaToraTah and PuToTyrannosarus Ranger Keys to reference to Super Hero Taisen.
- Technically, OOO has eleven offical Medal Combos, TaToBa, GataKiriBa, Latorator, SaGoZo,TaJaDol, ShaUTa, PuToTyrannosaurs, BuraWaKani, TaMaShii, TaToBa (King's Core Medals) and Super TaToBa, he would have twevle if the Black Core Medals had a Combo but Eiji never acquires them as they were destroyed during the battle against Kamen Rider Core in' 'Movie War Core with TaJaDol's Prominence Drop & CycloneJokerGoldXtreme's Xtreme Kick, if OOO had this unamed Combo, it would reference to his status as the 12th Heisei Era Rider.
- Ankh being disguised as Eiji in the World Arc in Angel Grove against Kazari was in fact, a reference to the Megamax movie where Poseidon lowers his guard after defeating Kamen Riders Birth and Proto Birth along with what he believed to be OOO, but Ankh reclaims a few Core Medals for Eiji to fight once more.
- In the World Arc, Eiji meets Kamen Rider Decade, the 10th Heisei Rider who is also the anniversary of the Heisei Era reaching it's 10th title, Eiji briefly meets Decade in the Let's Go! All Riders movie but his first personal meeting will be in Super Hero Taisen.
- Eiji also meets Kamen Rider Double, his predcessor in the World Arc as he first met Double in A to Z: The Gaia Memories of Fate taking on the Luna Dopant, this is also the first debut of OOO offically as he also joins forces with Double to take down Kamen Rider Core and for the third time, team ups with him in Let's Go! All Riders, Eiji would team up with Double a fourth time with Kamen Rider Fourze, the Kamen Rider after him in Movie Megamax and finally, Eiji will team up with Double for a fifth time and Fourze a third time in Super Hero Taisen, notably it seems Eiji has the most Rider Team-Ups but this is far beaten by Decade who meets all Heisei Riders before him, Amazon, Black, Black RX, Stronger along with all the Showa Riders in All Riders vs. Dai-Shocker, Movie War Core 2010 and Super Hero Taisen a total of 23 all together.
- Eiji's nightmare was in fact, Tokyo being brought to ruins like in Africa with slight differences, he no longer has the OOO Driver or any Core Medals and what caused the destruction was in fact, the Eiji Greeed which never have been controlled at all.
- Eiji never had a love interest since his desire was empty and even when he realises his desire for having the power to prtoect others, he never pursued a relationship, this may happen in Crossover Cove.