King (OOO)

The Ancient King, also known as the King, First OOO and Kamen Rider OOO, is the Greater-Scope Villain in Kamen Rider OOO and the main antagonist of Kamen Rider OOO: Final Stage, a mysterious king who becomes the very first Kamen Rider OOO. He is also an ancestor to Kousei Kougami and the predessor of Eiji Hino as Kamen Rider OOO, being the very first one who took on this title. His origin story was explained in the first part of Kamen Rider OOO novel, where the King serves as its main antagonist.

While he's dead long before the series starts, he's the cause of everything that's happened in the series because he had the Greeed and OOO created out of his own selfish desire to become a God.

History
The first Kamen Rider OOO was a king of a lost European kingdom 800 years ago, when the Greeed and OOO were created. He created the Greeed and the OOO Driver in order to absorb their power and become a god. He teamed up with Ankh to defeat the other Greeed, but ended up literally backstabbing him, only to die from being unable to contain all the power he absorbed, transforming him into the stone sarcophagus the Greeed were sealed in.

The prequel novel by Nobuhiro Mouri expands on his actions and shows that he cloned himself to attack neighboring countries; burned down villages that opposed him; invaded enemy camps using subterfuge; and used earthquakes and tsunamis to wipe out the Spanish Armada. After Ankh betrays the other Greeed by working with him, he double-crosses Ankh and stabs him in the back for his Core Medals and causes all the Greeed to be sealed away just so he could have the medals for himself.

After betraying Ankh, the King scanned all thirty Core Medal types at once in an attempt to gain their power through the OOO Scanner, which he thought could fulfill his desire to become a living god. However, this became his undoing. The power was too great for him to handle and the First OOO turned into the stone coffin, and the four Greeed: Uva, Kazari, Gamel, Mezool, along with Ankh's right forearm became sealed in. The OOO Driver, the very thing that the greedy king sought to gain power from, became the lock of the coffin itself.

Kamen Rider OOO: Final Stage
In the Final Stage, it is revealed that the King's soul was travelling in parallel worlds after his death. In order to regain his original power of OOO, he sealed every OOO in each parallel world and became the ruler of the World of PuToTyra. He uses a gold medal to teleport Eiji Hino, the last OOO from the World of TaToBa, a.k.a. the main timeline world, to the World of PuToTyra.

After meeting his successor, the King assumes the form of OOO PuToTyra Combo to gain the trust of Eiji, and reveals his true identity after gathering other Greeed. He battles Eiji in his Greeed form and gains an advantage until Ankh grabs the medals from him and the other Greeed. In the end, Eiji releases all the other OOO from the seal, and they fought together and defeated the King, ending his reign of terror once and for all.

Trivia

 * Rather fittingly, OOO can simply be pronounced as Oh, which is the Japanese word for King (王).
 * The King is completely different from Eiji: Whereas the former tried to use the power of OOO to take over the world, Eiji instead uses his strength to protect it, using the powers of the Medals to stop wars and give people hope for a new life.