Stiles's roommate at Eichen House. He has to be restrained at night. Oliver has been there for quite awhile, though it is unclear how long. The nogitsune is able to whisper to him and manipulate him, even while it isn't possessing Stiles.
It convinces Oliver to trepan Stiles and Malia and uses him to put Stiles into a situation where he will let the nogitsune in.
Oliver had swallowed a bug a few days ago and coughs it up. Once Stiles becomes the nogitsune, he knocks Oliver out and keeps him from hurting Malia.