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.
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.