Oma's Day

"Ohma's Day! The day of judgment marking the end of the Kamen Rider era. Is it the end of history or just the beginning? Choose our future!"

- Opening narration (episodes 17-29)

Oma's Day (オーマの日), alternatively spelled as Ohma's Day, is an anticipated event serving as one of the central plot points in Kamen Rider Zi-O, where Sougo Tokiwa became Ohma Zi-O. However due to certain events happening within the timeline, the progression of events that happened resulted in different timelines being created.

Prophesied Fight
Because of the multiple alterations to the timeline caused by several variables from the future, this led to Sougo's acquisition of the Zi-O II Ridewatch, which bestowed upon him the ability to have prophetic dreams of the future, with one such dream being a battle between Zi-O II and Geiz on Oma's Day. While his dreams did not reveal the outcome of the fated battle, the eventual winner would have decided which of the two timelines become reality, though such a battle did not take place in the original timeline before Sougo destroyed his Ziku-Driver and White Woz's arrival.

Original Timeline
"Sometime in the future, an event known as Ohma's Day happens. On that day, you became Ohma Zi-O and all the Riders aside from you disappear."

- Geiz explaining Oma's Day to Sougo In the original timeline, Sougo Tokiwa obtained the Zi-O Ridewatch II and gaining several more abilities to manipulate time as he wishes. Afterwards, he evolved into the demonic king Ohma Zi-O from his base form, destroying the world using the Dai Mazines, eventually wiping out all the Kamen Riders and at least 50% of the Earth's population. By 2068, Earth has become totalitarian dystopian where the Resistance attempted to fight back against Ohma Zi-O to no avail, causing Geiz, Tsukuyomi, and the Time Jackers to go travel back in time in an effort to change their future. However, Woz followed them through in an effort to stop them, although he later enters a feud against his white counterpart in an attempt to secure their own futures.

GeizRevive's Timeline
"On the upcoming Ohma's Day, you will destroy Ohma Zi-O, and become the one to change history!"

- White Woz revealing the newly diverged Oma's Day to Geiz and Sougo Due to the present day Zi-O temporarily giving up his Ziku-Driver, it created a diverging future where Geiz obtained the power of GeizRevive and defeated Ohma Zi-O, later becoming knwn as the world's savior. Sometime after this day, the Woz of this timeline traveled back to January of 2019 to assist Geiz and ensure that this timeline is the one that runs, eventually entering a feud with his black counterpart to secure their timelines. Due to this, new Kamen Riders are able to rise, as such Kamen Rider Quiz in 2040. White Woz describes this timeline to be "at peace, like time itself has stopped".

It is never explicitly stated whenever Sougo is alive or not in this timeline.

Zi-O II

 * Sougo Tokiwa travels to the Mirror World and meets his Mirror world self, who torments his destiny as Ohma Zi-O. Sougo is able to overcome this tormenting and fuses with his Mirror World self, obtaining Zi-O II instead of becoming Ohma Zi-O. Because this happened, Ohma's Day was eventually pushed back to April 7, 2019, the day the brightest star in the Leo constellation, Regulus shined at its brightest point, as noted by both Black Woz and Swartz.

The Fated Duel Averted

 * Heure helped Black Woz steal the Kamen Rider Woz powers from White Woz in an effort to delay the sequence of events that White Woz had originally envisioned.
 * Geiz attains his GeizRevive form much earlier than prophesized, thus his continued over-usage of the form made him weaker to the point where he may not survive.
 * By becoming Kamen Rider Woz, Black Woz used its powers to stop Geiz from entering into a battle with Sougo, as well as warning him of GeizRevive's dangers.
 * After surviving 2009, Tsukuyomi returns to 2019 with news that the future can be changed, which resulted in both Sougo and Geiz setting aside their differences and reconciling. Thus, the supposedly fated duel between the two Riders was averted.

Achieving Zi-OTrinity

 * With his plan foiled and being unable to secure his own future, White Woz became fed up and enlisted the help of Kamen Rider Diend to manipulate Amane Kurihara, Kazuma Kenzaki/Kamen Rider Blade, and Hajime Aikawa/Kamen Rider Chalice out of hiding and restarted the Battle Fight for the purpose of stealing both the Zi-O II and GeizRevive Ridewatches in order to carry out his own plans to destroy the world completely.
 * Due to Sougo being able to convince White Woz not to give up fighting for his own future, White Woz willingly abandoned his plans, though the Battle Fight was still ensuring due to his past actions. This led to them fusing their Rider powers, resulting in the creation of Zi-OTrinity on Oma's Day. With White Woz being erased from existence after his future was negated, Oma's Day was pushed back again by another four months, culminating in a series of events unique to this variation of the timeline.

The Path to GrandZi-O and Beyond...

 * Because Sougo was still on track to become Ohma Zi-O sometime in the future, his future self reveals that there were still six Ridewatches left to unlock before Sougo could fully achieve kingship.
 * Throughout his journey to collect the remaining Ridewatches, Sougo had met several past Riders as well as defeated their Another Rider counterparts. With the acquisition of the Den-O Watch, it fully created the GrandZi-O Watch, which Sougo recklessly took to 2068 to fight Ohma Zi-O with. However his future self was still stronger due Sougo not faithfully having Drive's Watch, as Geiz previously stole it. This allowed Hiryu Kakogawa to create a massive time distortion in 2019, jeopardizing both of their existences.
 * In the altered time distortion, Swartz is successful in stealing Tsukasa Kadoya's powers and becomes Another Decade, later stripping Ora and Tsukuyomi of their powers and disbanding the Time Jackers.
 * The altered time disruptions forces Michal Minato to intervene from 2050, revealing that Tsukuyomi and Geiz's good intentions are the actual causes of the coming apocalypse they've been trying to prevent, since their presences in the timeline are causing a for Ohma Zi-O's ascension.
 * The team later meet with Go Shijima and Krim Steinbelt to acquire the Drive Ridewatch, and face Quartzer where they learn of Black Woz's true origins.

The Path Towards Ohma Zi-O

 * With Swartz's acquisition of Another Decade, he summons the Dark Riders from the Another Worlds, who attack the team relentlessly, resulting in Michal Minato's death.
 * Swartz presents Sougo with an ultimatum: choose to save himself, save Tsukuyomi, or save everyone by becoming Ohma Zi-O.
 * As the timeline begins to break down in stability, the Ridewatches crack and the worlds begin to merge together, resulting in various landmarks, monsters, and revived people appearing in the city.
 * To fix this situation, Tsukasa takes Tsukuyomi and Sougo to 2058 in order to find out how to un-merge the worlds, though the negotiation goes south, and teen Swartz kills Tsukasa. Daiki Kaito arrives and brings Tsukasa back using Hiryu's Another Watch, though it came at the cost of turning himself into Another Zi-O II and mindlessly attacking Decade. After a quick battle that results in the Another Zi-O II watch being destroyed, Tsukasa announces his intention of destroying the world upon seeing how hopeless it was.
 * Tsukasa presents a contingency plan to Sougo and others, which they agree on and help fight Swartz in the midst of the chaos.
 * Sougo returns to 2068 and presents his dilemma to Ohma Zi-O, who against his own judgement, chooses to help his past incarnate and creates the Tsukuyomi Ridewatch for Tsukuyomi to use in 2019.
 * Tsukuyomi transforms, but she quickly turns traitor to the team and swears loyalty to Swartz, marking the beginning of Tsukusa's plan to have two Riders from different worlds create a temporal bridge that connects both their world and Tsukuyomi's.
 * Junichiro repairs the Ridewatches, which restores Sougo's GrandZi-O Ridewatch. He uses his returned power against Swartz in their final showdown, however Swartz is victorious and kills Geiz, triggering Sougo's transformation into Ohma Zi-O.
 * With this newfound power, Sougo mercilessly one-shots the summoned villains and kills Swartz, later prompting to rewrite history rather than rule it. At this point in time, Ohma Zi-O disappears from existence as peace is restored to the world.