Synopsis from wikipedia:
Eiji Hino is a traveling man who has no place to call home and a tragic past. When coin-based homunculi known as the Greeed awaken after their 800-year slumber to attack humans and feed off of their desires, the disembodied arm of the Greeed named Ankh gives Eiji a belt and three Medals to fight the other Greeed as Kamen Rider OOO. The mysterious Kougami Foundation approaches Eiji and begins assisting him in his fight against the Greeed, though their true motives are not clear. As Eiji fights the Greeed and their Yummy monsters, learning more of the Greeed and Ankh, he starts to find a purpose beyond his journey.
In OOO there are two types of medals: Core Medals, that are the essence of the Greeed and can be used in OOO's belt and Cell medals, that give form to the Greeed. The Greeed can insert cell medals in humans in order to bring out a Yummy, a monster that personifies the desire of a human. The Yummy satisfies the human's desire which generates Cell Medals for the Greeed. The Yummy does everything to fullfill a desire, including hurting people and its own host. They are also used agains OOO and Ankh, since the latter steals Core Medals from the other Greeed in order to handicap them, since he himself is handicapped. When the Yummy dies, he releases all his Cell Medals, which not only the Greeed, but the Kougami Foundation is after. Each Greeed has a theme and a color for his medals: Uva's (green) is insects, Cazali's (yellow) is felines, Mezul's (blue) is aquatic animals, Gamel's (silver) is big mammals and Ankh's (red) is birds.
OOO's design is good, but not one of the best among riders. He uses 3 Medals to transform, one for the legs, one for the torso and arms and one for the head. The medals are specific, so a torso medal can only be used in the torso. The same goes for the Greed. Each one of them has three leg medals of the same animal, three for the torso and three for the head. Each type of medal gives OOO a different ability. Grasshopper legs make him jump higher and cheetah legs make him run faster. His fighting style also changes with the medals. The suit actor did an awesome job with this. The vending machines that became the bikes were ok, but weren't used much. There were also candroids, small robots used as spies or communication devices and were similar to Hibiki's disc animals. They dind't really make much of a difference. Just more toys to sell, I guess.
With all those powers, OOO had the potential to be a rider that used strategy in his fights. Unfortunately, it didn't happen. He relied on combos (using three medals of the same color) or willpower to win. Overall, the fights were pretty bland because of that, even though the suit actor had some nice moves. The second rider didn't help with that and spent most of the series as a sandbag. The giant enemies were unnecessary (one of the worst Heisei rider tropes) and the 2nd rider's mecha was totally lame.
Eiji is a very good rider. His personality is similar to Kenshin's (Rurouni Kenshin). He doesn't care for money and status, being satisfied with having underpants for tomorrow. He also knows that saying he fights for justice or other ideal might lead him to end up doing harm to others, so he just fights to protect those that are in front of him, even though his excessive self-abandonment is also a danger. Ankh, as a Greeed is very ambitious, but since he's just a hand, he wants to recover his power ASAP. He's arrogant and anti-social, but also very clever. He's always trying to trick people and the Greeeds to get what he wants. The relationship he has with Eiji is very interesting. Eiji wants to save people and Ankh wants medals. They both try to use the other while being ready to fight if it comes to it. At least in the beginning. The last part of the main trio is the heroine Hina Izumi. She's the sister of the policeman that Ankh possesses (because he didn't want to just float as an arm all the time). She's a sweet girl, but she's super strong. They don't explain why and this strength is never used against the villains, so it was kinda pointless. Even though she's important, she's not well developed and ends up being bland. There's also Chiyoko Shiraishi. She's the owner of the restaurant in which Hina and Eiji work part time. Eiji and Ankh live in the back. She serves food from all around the world and makes everybody cosplay as foreigners or characters to bring customers in. She's there pretty much just to give Eiji a job and a place to live. She isn't important at all in the plot, even though she has a minor part near the end.
The Greeed look very cool, but unfortunately, they aren't well developed and are just complete monsters that had no goal except getting stronger. And the Greeed could've been much more. For instance, they waited until the end, when one of the Greeed was going to die, to reveal that it wanted a family. Only Cazali shows a little more, since he's very clever and acts as a rival for Ankh, because the other Greeed don't really make plans. There is another villain who is an interesting character, but he was another one that wasn't well used. Even though he had power knowledge and a cool backstory, he was never that villain that you hate or that makes you think: oh shit, here he comes! Another problem with the villains was the Lucky Clover Syndrome (named after the KR 555's villain group). The Greeed, who are the ultimate enemies, go to the battlefield too soon. Since both the Greeed and the heroes have to last until the end, the power levels have to keep changing to match what's necessary at the moment. One week a Greeed wipes the floor with the heroes just to be pushed into a corner by the same heroes in the next week, even if the heroes didn't get a new power. The Greeed also had to keep leaving the fight when they have the advantage, instead of just destroying the heroes, or having to run away constantly. The medals were also won and lost pretty easily.
On Kougami's Foundation side we have Kousei Kougami, the president, who is fond of beginnings and is always making cakes to celebrate the birthdays of people and stuff. He randomly shouts Happy Birthday! when something new happens. He also admires people with desire. Shitaro Goto is a former policeman full of pride that joined the Foundation to save the world, so he doesn't like Eiji and Ankh very much. However, he is gets a lot of character development and ends up being a good character. Erika Satonaka is Kougami's lazy secretary and cake taster. She's cool and tough. There's a character that comes later in the story that's very good. He acts as a mentor for Gotou and as a friend for Eiji. Unfortunately, the Foundation itself is kinda pointless and is there more to be the shady organization that must exist in a KR series.
Some of the 2-episode arcs had very interesting stories. They showed the consequences of people's pure desires becoming reality without any restriction. For instance, a man's desire for justice creates a violent Yummy that hurts people for the smallest of errors. Those reflections on the nature of human desire were one of the show's best points. Unfortunately, the main plot was very simple and mediocre.
OOO is a KR series with a lot of good points, but a lot of problems. It's not a bad series, but it could've been far better than it is.