The oni are creatures that live in the Cloverfields beyond Shomonkai. It is suggested that they have a nest there but that can't be determined for sure as we haven't been able to search as far. They are extremely hostile and attack in packs. Scientific analysis shows they are attracted to Reiki and are also able to feed on it, replenishing and boosting their strength while doing so.

Oni are divided in seven ranks according to their strength, starting with level one -the weakest- to level seven -the highest-. Oni of the sixth level can adopt a form that is closer to that of a human, the only thing betraying them being their eyes when they are using their powers in any form or shape -even if it simply their dairokkan reaction to a Spirit with a high level of Reiki. Even so, Oni of the sixth level can adopt the same powers Spirit people do and are even able to wield a mutated form of Reigen -because their ki is impure.

Oni of the fifth level are almost as smart as sixth level ones, however they lack the ability to transform into a human-like form and cannot use any other powers like the next rank can. They are still exceptionally powerful creatures. Oni of the fourth level and bellow only become less smart and powerful as their rank decreases, with level one Oni being much like animals in terms of intellect.