|This article is about a/an item in Kamen Rider Zi-O.|
The Oma Advent Calendar (逢魔降臨暦 Ōma Kōrin Reki), alternatively spelled Ohma Advent Calendar, is a book from the year 2068 which details the rise of the overlord Oma Zi-O, whom supposedly began as Sougo Tokiwa. It is held and consulted by Oma Zi-O's servant and prophet Woz, who seeks to preserve the history of his master. This book is written in English. This book can even display moving images of Zi-O and his actions. Woz is able to use this book for combat up to a limited extent, using it to temporarily entrap his targets. When he is attacked, Woz appears to "bleed out" pages of this book.
Through this book, Woz was aware that Sougo Tokiwa first transformed into Kamen Rider Zi-O, which Woz viewed as Sougo's first step to one day becoming King of the world. Traveling back to this day, Woz told Sougo that it would be a momentous one for him, but that he must beware the "red robot". Indeed, after Woz appeared, Sougo was attacked by the Time Mazine, a red-colored giant robot piloted by Geiz Myokoin, whom also travelled back from 2068. When Sougo unlocked the Zi-O Ridewatch, Woz appeared before him again and urged him to use his new power to shape the world to his will. Sougo, having been made aware by Tsukuyomi that Oma Zi-O was the most despised tyrant in history, vowed to instead become the greatest and most beloved king. Though this came as a surprise to Woz, he nevertheless provided Sougo with the Ziku-Driver which enabled the youth to achieve his fated transformation into Kamen Rider Zi-O.
Following Zi-O's first battle, Woz informed Sougo that he was destined to rise as the ruler of time through a path of conquest. He also warned him of the Time Jackers who sought to impede his ascension in an attempt to groom others to become king. Soon after, Woz informed him that the emergence of Another Build has altered history, usurping Sento Kiryu as Kamen Rider Build. He warned Sougo that, even as a potential overlord, he was yet to acquire the ability to defeat Another Build, who was being engaged by Geiz in the meantime. Traveling to 2017, Woz caught up with Sougo as he accessed the Build Ridewatch, advising him that victory would include a great deal of responsibility. It was at this point that Sougo used the Ridewatch to assume the BuildArmor, commemorated by Woz as the first time Zi-O would receive the power of another Kamen Rider.
In Kamen Rider Zi-O: Supplementary Plan, the Oma Advent Calendar serves as Woz's script.
Behind the scenes
As the chronicle of Zi-O, it is from the Oma Advent Calendar that Woz reads the recap of each prior episode. Likewise, previews for the next episodes are projected from its pages.
The design of the Oma Advent Calendar was adapted into a Buttobasoul Medal Holder.