Oni-oh emerges as a colossal red oni, an unintentional fusion of over a thousand oni spirits catalyzed by Kyōyama Anna's uncontrolled spiritual energy forcing a Naka-Oni's rapid evolution. Bound by chains of resentment, these interconnected spirits grant Oni-oh limitless power and remote manipulation of its components.

Distinguished by towering stature and horns, it exhibits intelligence surpassing ordinary oni, enabling complex speech, strategic combat, and psychological manipulation. Despite sadistic inclinations, it speaks with measured calm and recognizes Anna as its creator, addressing her as "mother." Oni-oh evolves by consuming other oni, gaining regenerative abilities and enhanced strength with each absorption. These consumptions trigger visible transformations, including a thick mustache and manifested clothing.

Its combat repertoire includes the Oni Drum Roll—summoning smaller oni from arm stumps—and replicating Anna's signature attack, the Phantom Oni Left. Defeated by Asakura Yoh, Oni-oh is banished to Hell. After initial struggles, it allies with Sati Saigan, shifting roles to guide Hell-bound trainees.

Later integrated into the Great Spirit's escort for Hao's mother, Oni-oh resurfaces when Shaman King Hao implants it into Asakura Hana's body. Compelled during a lethal clash involving Ichihara Ryuji, it manifests physically and fights until a shrine spirit's beam beheads it. Fragments merge with other oni to form Daidarabotchi. An identical copy emerges when Alumi Numbirch summons Cloud Anna, replicating the Phantom Oni Left.

In ancillary media, Oni-oh appears as the "Great Ogre" boss in *Shaman King: Master of Spirits 2*, retaining its combat color shift from blue to red. Promotional materials depict a non-canonical spirit ball design.