The Oni (Japanese: 鬼, lit: 'ogre') are a race of creatures who have existed since primordial times and were once engaged in a great war with the Genma. After losing this war, the Oni suffered considerable losses and their remaining numbers were scattered. Because of this, few Oni are actually featured in the series - although their power is invested into their human allies. The Oni represented are similar in appearance to their mythological origin: They are beast-like humanoids with horns and wild white hair. These characteristics are often transferred to the human warriors during their Onimusha transformation.
Although not as diverse as their Genma counterparts, there is some variety to the Oni clan. Takajo from Onimusha 2: Samurai's Destiny, actually has a serpentine appearance and there are depictions of similar Oni in the Onimusha 3: Demon Siege files.
In the case of the twelve Oni Gods who appeared briefly in Onimusha: Warlords, we can deduce that was their spiritual forms and not their physical appearance.
Science of the Oni
Little was known about the Oni civilization until Onimusha 2, when Jubei encountered several examples of advanced technology. This includes a mechanical horse, which he and Oyu used to escape Gifu Castle, and the magnificent flying fortress called 'Oni Genbu' which is housed within the Oni Sacred Place. Before that, the only visible relic of their power was the Oni Gauntlet, which absorbs the souls of demons and controls powerful magic weapons. In Onimusha 3, it was also possible for the Oni to seal an entire army within an orb designed for the gauntlet.
The other notable accomplishment of the Oni race is their ability to create temporal distortions, such as portals.
- In Warlords, a note written by Sougen Osho they're called by their Japanese name the "Ogres", but for the rest of the series their called the "Oni".