Three Oni minifigures appear in 853866, and share the same printed heads and torsos. Two of them reuse the Black Panther head topper piece, but turned around so that the "ears" become horns. All three have plain black legs, with one set being the short variety with no joints. One Oni can also be fitted with a motorcycle helmet and a recolored version of the Oni mask parts from 70643 Temple of Resurrection. The black horn elements that go with this piece were first introduced in 70795 Mask Maker vs. Skull Grinder. One of the Oni wears the shoulder armor originally introduced for Sons of Garmadon characters like Mr. E, while the other two wear the armor with a weapon slot introduced for the Ninja in 2015.
In the animated series, the Oni differ in appearance from the physical minifigures, with the printing from their torsos continuing onto their legs. Some of the Oni also have legs other than the standard minifigure legs, instead looking more like the legs of some kind of animal such as a goat. Rather than using the Black Panther head topper, the TV Oni-including Mystake-feature a large horn piece that has not been produced physically. Many of the TV Oni also have the same head/mask piece as their leader, the Omega, but with smaller horns; some of these also wear ragged capes like the Omega.
The Oni originated in the First Realm, where they were bitter enemies of the Dragons. When the First Spinjitzu Master with equal measures of the Oni power of destruction and the Dragon power of creation, the two races fought to possess him. Eventually, the master fled the First Realm to escape the conflict, after failed efforts to bring peace between the two factions, and eventually brought Ninjago into being. However, the Oni were unwilling to give up on pursuing him, and managed to send a team of Oni to Ninjago to capture him. However, the Oni Mystake came to love the new world, and aided the First Spinjitzu Master in defeating her companions before settling in the realm.
Through unknown means, the three Oni Masks of Vengeance, Deception, and Hatred-each representing a different Oni warlord-made their way to Ninjago as well. When brought together to the Temple of Resurrection, they had the ability to open a portal to the Departed Realm and bring back Lord Garmadon. While in the Departed Realm, Garmadon apparently witnessed the Oni making their way to Ninjago, and thus took over Ninjago City with his Colossi and the Sons of Garmadon in an effort to prepare for their arrival. The Ninja, upon traveling to the First Realm, found the Oni absent with no trace of them but ruins, though the Iron Baron and his Dragon Hunters were seeking a means of combating them. The Oni were infamous for their shapeshifting ability, and once decimated a party of Dragon Hunters that came looking for the Dragon Armor.
Upon being defeated by Lloyd, Garmadon warned the ninja that the Oni were coming. Some time later, the Oni launched an attack on the former Dragon Hunters in the First Realm, and despite their alliance with the Dragons overcame them, with only Faith and the Firstbourne managing to escape to Ninjago. The Oni then use the Realm Crystal to open a portal to Ninjago themselves, and soon envelop Ninjago City in a cloud of darkness. Any citizens caught within the darkness become locked in a deep sleep, and the nature of the cloud is such that no human can remain inside it for long without succumbing. As a result, the Ninja are forced to recruit Garmadon to help them against the Oni, with him proposing a mission to destroy the Realm Crystal.
Joined by Lloyd, who due to being part Oni is able to survive the cloud, Garmadon makes his way to Borg Tower and engages a pair of Oni guarding the front doors. Once inside, the pair encounter the Omega, and in a brief struggle are able to shatter the Realm Crystal. However, with their portal open the Oni no longer require the artifact, and the Omega summons more Oni to mob the pair. They are able to escape after locating the armor of the Golden Master, which the Oni recoil from in fear. Despite this, more of the creatures converge on them outside, and it is only the arrival of P.I.X.A.L. in her Samurai X Mech that allows them to escape.
The Oni quickly pursue the Ninja to the Monastery of Spinjitzu, and as an army engage the Ninja, Garmadon, Faith, and Master Wu. Despite the Ninja's use of the reforged Golden Weapons and Garmadon's own Oni powers, the sheer numbers of the Oni force them back. However, Lloyd then hits upon the idea of performing the Tornado of Creation, as Creation is the natural opposite of the Oni power of Destruction. The Ninja, Wu, and Garmadon do so, and upon awakening from the burst of energy find that the Oni have vanished, with the darkness likewise disappearing.