Are you looking for a medical (psychiatric) term, or are you looking for a word laypeople know? Anyway, candidates include:
- 白昼夢: "daydream"; a dream seen while awake
- 夢想(的): "dream; vision"; basically a stiff kango version of 夢
- 幻覚(的): "hallucination"; a psychiatric term known to laypeople; usually not associated with fantastical elements or decreased consciousness level
- 幻想(的): "fantasy" as shown in this picture
- 夢幻(様): a rare psychiatric term used to translate oneiroid state; but it's also widely used with no clear meaning in various fictional contexts
- サイケデリック: "psychedelic" like this
- 譫妄: "delirium", a common psychiatric term
If you are looking for the best Japanese medical term to describe oneiroid state or oneiroid hallucination as described in English Wikipedia, I would choose 夢幻様症状 or 夢幻様体験. But the concept itself seems to be little-known, and it may be categorized simply as 譫妄 today.
If you are looking for a word everyone understands, I believe there is no established translation, so you have to choose one of the above words depending on your purpose. If vividness is important, 幻覚 or サイケデリック may be used. If the fantastical aspect (e.g, unicorn, fairy, ...) is important, 幻想 may be better. Otherwise simple 夢想 may be the safest. Can you name a few films which you think is oneiric?