Oma's Day

"Oma'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) is an anticipated event serving as one of the central plot points in Kamen Rider Zi-O, which occurred on 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.

However, due to the present day Zi-O temporarily giving up his Ziku-Driver, the expected progression of events that led to Oma's Day differed across two divergent timelines. The Time Jackers themselves were trying to prevent both timelines from happening by creating an Another Rider powerful enough to stop both Zi-O and Geiz and have it become king instead. However in the end, a unique version of Oma's Day occurred in the present timeline, culminating in the formation of Kamen Rider Zi-OTrinity and leading into a new future which remains to be known.

However, according to Michal Minato, due to the actions of Geiz Myokoin, Tsukuyomi, and the Time Jackers interfering with events in the timeline caused a temporal loop that indirectly created the conditions for Oma Zi-O's ascension.

Possible timelines
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.

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

- Geiz explaining Oma's Day to Sougo According to Black Woz, on this day, Sougo used the Zi-O II Ridewatch to make his grand transformation into the demonic king Oma Zi-O and destroyed the world using Dai Mazines, eventually wiping out all the Kamen Riders and at least 50% of the Earth's population. This global catastrophe created the dystopian timeline where the Resistance attempts to fight back against Oma Zi-O to no avail, causing Geiz Myoukouin, 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 Oma's Day, you will destroy Oma Zi-O, and become the one to change history!"

- White Woz revealing the newly diverged Oma's Day to Geiz and Sougo On this day, Geiz defeated Oma Zi-O and became known as the world's savior, later evolving into GeizRevive. Sometime after this day, the Woz of this timeline traveled back to January of 2019 to assist Geiz and ensure the timeline runs the way he wants, eventually entering a feud with his black counterpart to secure their timelines. Due to this, new Kamen Riders are able to rise, such as Shinobi in 2022 and 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.

Fated Duel Averted
Several factors in the timeline had eventually caused the fated duel not to happen.
 * 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 steal the Zi-O II and GeizRevive Ridewatches in order to carry out his own plans to destroy the world completely. By forcing Amane Kurihara to turn into Another Blade, he weeded Kazuma Kenzaki/Kamen Rider Blade and Hajime Aikawa/Kamen Rider Chalice out of hiding and restarted the Battle Fight between the Joker Undead anew. As he grasped the Zi-O II and GeizRevive Ridwatches while it resonated with the innate Kamen Rider Woz power within him, he realized that a new power could be born from the three. This led to the fusion of the three Rider powers, resulting in the creation of Zi-OTrinity on Oma's Day. With White Woz erased from existence, his future having been negated, a new and unknown future is waiting to be seen.

Oma Zi-O's Message
However Daiki Kaito reveals to Sougo and his friends that Oma Zi-O is still in fact intact in the timeline, which meant that Sougo was still on track to become Oma Zi-O sometime in the future, albeit much later than originally. Oma Zi-O reveals that there are six remaining Ridewatches left to unlock.

Achieving GrandZi-O
With the acquisition of the Den-O Watch, it resonated alongside all the Ridewatches, merging together into the GrandZi-O Ridewatch. Sougo used it to defeat Another Den-O, but was soon pulled into 2068 by Oma Zi-O, who was still stronger because of having every single Ridewatch with him. Upon GrandZi-O's protest, Oma Zi-O began attacking him with Blade and Double's powers, triggering a duel, hence creating a massive time distortion in 2019.

Conflict with the Quartzers
To be added.

Swartz's Three Choices & Final Showdown
To be added.