|This article is about an item in Kamen Rider Zi-O.|
|"According to this book, today will be a very special day for you."|
|―Woz to Sougo Tokiwa[src]|
The Ohma Advent Calendar (逢魔降臨暦, Ōma Kōrin Reki), is a book from the year 2068 which details the rise of the overlord Ohma Zi-O. It is held and consulted by Ohma Zi-O's servant and prophet Woz, who seeks to ensure that his master comes to power. It is written in English. 「GO! GO! Ghost 2015」 The pages can also display moving images. 「2019: Missing World」 Woz is able to use the Ohma Advent Calendar in combat to a limited extent, as it is able to temporarily contain his targets. When he is attacked, Woz appears to "bleed" pages of this book. 「The Beginning of Everything 2009」
In Kamen Rider: Reiwa The First Generation, he owned an alternate version of the book named as True Ohma Advent Calendar (真逢魔降臨暦, Shin Ōma Kōrin Reki). Compared to the original, the text in this is written in Japanese.
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 Ohma 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. 「Kingdom 2068」
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. 「Best Match 2017」
Following history's alteration in the finale, the book became blank. The only text in it is a single page which reads "A new generation. The first Rider shall be born." (新時代。始まりのライダーが誕生する, Shin jidai. Hajimari no raidā ga tanjō suru). 「Kamen Rider: Reiwa The First Generation」
Behind the scenes
As the chronicle of Zi-O, it is from the Ohma 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 Ohma Advent Calendar was adapted into a Buttobasoul Medal Holder. The book is also used for the CD Box Set, containing the collection of theme songs, all of the soundtracks from the 'TV Series' and both movies featuring Zi-O. The book is again used for the Zi-O Chozenshu King's Box which also included the DX Bibill Ridewatch, in this instance being a photobook of the Riders and forms shown throughout the series and movies.