One night, while looking in the mirror he began to hallucinate that he could not see his flesh or his bones.
The episode then dives into A Christmas Carol territory as Oliver starts to hallucinate.
c.1600, "deceive," from Latin alucinatus, later hallucinatus, past participle of alucinari "wander (in the mind), dream; talk unreasonably, ramble in thought," probably from Greek alyein, Attic halyein "be distraught," probably related to alaomai "wander about" [Barnhart, Klein]. The Latin ending probably was influenced by vaticinari "to prophecy," also "to rave." Sense of "to have illusions" is from 1650s. Occasionally used 19c. in transitive senses, "to cause hallucination." Related: Hallucinated; hallucinating.