According to Sammobile, it seems that Samsung has already began preparations for the roll out of an Android 8 Oreo beta program for their Galaxy S8/S8 Plus models.
The website suggests that they have already come across newer Oreo firmware versions for both devices: G950FXXE1ZQI7 and G955FXXU1ZQI7 and the letter “Z” in the names, suggests that they belong to test firmware. The same type of firmware was spotted on the beta Nougat releases for the Galaxy S7 and Galaxy S7 edge last year.
So far that’s the only info they have available, along with the news that Samsung’s skin is seriously delaying the whole procedure, as it is really heavy in terms of resources and usability. The same information from Sammobile suggests that the Galaxy Note8 will also get the Oreo update really soon, but still this won’t happen until the Galaxy S8/S8 Plus models get theirs first.