Queen developed Ice Oni specifically as an enhancement for the Pleasures and Waiters, who were otherwise replaceable and had no use in the crew due to both groups losing their chance of being granted SMILE fruit powers.
The virus is fused with a chilling gas, and once this gas is spread through the body the virus is able to reproduce in it, causing the victim to feel extremely cold as ice covers their skin. The ice will take on a demonic shape, causing a pair of horns and sharp fangs to manifest on the victim's head and mouth, giving them the appearance of an oni.
Once their body is covered in ice, the victim appears to lose their sense of reasoning and enters a berserk state, targeting anyone around them, ally or not. Moreover, they receive a boost in physical strength and durability due to an increase in muscle mass (in the case of Hyogoro, he regained the physiology from when he was at the peak of his strength), meaning they are nearly impossible to defeat through conventional means. Zoro compared their endurance to that of Zombies, as they are able to immediately get up after being struck down. According to Chopper, the virus's symptoms cause too much strain on the victim's body, leading to their eventual death in after around an hour at best following the initial infection, as confirmed by Queen.
This virus can quickly spread from person to person through physical contact, with one victim being shown biting an uninfected person to transmit it. Because of this, fighting back the infected requires one to use a weapon or another object to avoid physical exposure to the virus. Brook came into contact with the virus, but did not suffer its effects since he did not have any skin or blood.
The only way to cure Ice Oni is through an antibody created by the virus's developer himself, Queen, who happens to have been in possession of the only known existing sample. With access to Ice Oni's antibodies, Chopper was eventually able to craft a countervirus in a short amount of time, releasing it in mist form through cannon fire among a crowd of victims. This was considered a more practical cure than mass-produced liquid antibodies, since the Chopperphage's nature as an air-travelling organism meant it could be inhaled by several people at once, thus combating Ice Oni's quick spreading.
Additionally, the progress of the virus through the body can be halted if the victim stands next to a source of fire, as the heat will eliminate the cold gas and prevent the virus from reproducing. Marco's Devil Fruit ability can reverse the transformation by increasing the victim's body heat to a point that the frost disappears, but this is only a temporary solution as the virus still lingers within the victim, resuming the infection process once their stamina runs out.
Believing that the Pleasures were defective soldiers to the crew, and once the supply of SMILEs was severed, which meant the Waiters lost their chance to become Gifters, Queen developed the Ice Oni virus in order to make these two lowest-tier divisions more useful for the crew, if only for a short amount of time at the cost of their lives.
Four Emperors Saga
Wano Country Arc
When war broke out at the Live Floor in Onigashima between the Ninja-Pirate-Mink-Samurai Alliance and the Beasts Pirates and X Drake changed sides, Queen fired Excite Bullets from a Gatling gun at the enemy samurai, intending on closing off the room and spreading the virus to everyone in it.
Queen informed everybody in the Live Floor about their condition and how the Ice Oni would eventually kill all the infected within an hour, tossing the only known vial of the virus' antibody to Scratchmen Apoo so everybody would fight each other over it. Chopper became infected as well, but not before realizing that the virus could be temporarily suppressed with heat.
Zoro and Drake teamed up against Apoo for the antibody vial, with Zoro managing to knock the On Air pirate out and retrieve the vial before handing it over to Chopper. Marco eventually entered the Live Floor and provided temporary relief for the victims with his phoenix flames, allowing Chopper extra time to develop a cure alongside Miyagi and Tristan.
When the Pleasures and Waiters pleaded to Queen to save them from the virus, he coldly rebuffed them, informing them of the pathogen's purpose, resulting in many of them defecting from the crew in protest. Chopper, who managed to analyze the small sample of the antibody, used it to create a gaseous variant, the Chopperphage, so he could completely cure the entire floor's worth of victims quickly.
- This virus is named after the children's game "freeze tag", which is known in Japan as "Ice Oni" (こおり鬼 Kōrioni?).
- The effects of Ice Oni resembles the traditional depictions of a zombie apocalypse in fictional media, which sometimes have highly infectious viruses act as the catalyst for it.
- One Piece Manga — Vol. 98 Chapter 993 (p. 5-6), Ice Oni is first seen.
- One Piece Manga — Vol. 100 Chapter 1007 (p. 4-5), Queen reveals the true reason of the Ice Oni.
- One Piece Manga — Vol. 99 Chapter 997 (p. 10), Chopper explains how the virus works.
- One Piece Manga — Vol. 98 Chapter 994 (p. 10-13).
- One Piece Manga — Vol. 99 Chapter 995 (p. 12).
- One Piece Manga — Vol. 99 Chapter 998 (p. 2-5).
- One Piece Manga — Vol. 100 Chapter 1006 (p. 8).