The Oni are demonic warriors who at first appear neutral, but are later the secondary antagonists in Season 3B of MTV's Teen Wolf.
They are often summoned and ordered to carry out certain missions and tasks and often do so without any kind of personal motivation. They are described as a force of nature compared to a tsunami and will often do anything and stop at nothing to complete a task. They have also been known to kill anyone who opposed them.
The Oni first appeared when the Nogitsune was unleashed. They were summoned and controlled by the breaking of special knives which are apparently the physical manifestation of the Kitsune, Noshiko Yukimura's tails. Noshiko used them to find the Nogitsune and they would mark anyone who wasn't possessed by it. It is revealed Chris Argent once encountered the Oni when he was young at a arms deal with the Yakuza. Chris, Isaac, and Allison had later met with a Yakuza gang member who was at the arms deal and he warned them, saying that no man made weapon can kill The Oni.
After discovering that the Nogitsune possesses Stiles, the Oni spent a lot of time hunting down the Nogitsune however, it was separated from Stiles, but assumed his form. The Nogitsune later managed to gain control of them by gaining possession of one of the Kitsune knives and breaking it, using Lydia to sense when they were coming. Before Allison died, she revealed that their weakness was silver and could be killed by one, as she killed one with a silver arrow . Scott and the rest of the crew used this to their advantage and managed to kill most of them as they stopped the Nogitsune.
One later appeared when Kira and her mother went to the New Mexico desert where the Skinwalkers lived and was teaching Kira how to control the fox spirit within her. This Oni was defeated while the Skinwalkers were testing her.