Boy oh boy Hear me out The first isekai is... THE BIBLE

Depends on a few things. For the movie, most of the characters in Oz are the same as characters in the real world and we see Dorothy waking from a head injury. If SAO isn't Isekai, then Wizard of Oz is not either. Then there's the matter of the Wizard being a man from Kansas as well but arriving by hot air balloon points to Oz being a place on Earth, so not another world. Therefore not isekai.