Oni perform a scan on any species of being - human or supernatural. This process is used to identify any individual that has a Dark or malevolent spirit attached to them.

                                                                                                                                      

Oni perform the test by looking into a suspect's eyes in an almost ritualistic fashion. If the said individual is not possessed, they are marked by the Oni on his neck behind the left ear with the Japanese kanji "Onore" (己) which means "self". This is to signify the suspect is still themselves. The process leaves the suspect incapitated in a state of hypothermia.

 

In Illuminated, all the supernatural beings in Scott's Pack (Isaac Lahey, Lydia Martin, Ethan, Aiden and Derek Hale) were scanned and marked. Afterwards, in Silverfinger, Scott and Kira Yukimura allow themselves to be scanned and are marked themselves.

 

Stiles, who was possessed by the Nogitsune, allows the Oni to test him in Insatiable, to make sure he is completely exorcised after the Void Kitsune splits from his body and copies his shape. The process works and he is marked with self.