Has the OEM in question been revealed yet? Likely not one of the major OEMs because they all lock their bootloaders. I'm crossing my fingers it's Fairphone but that's because I love my FP5. The GrapheneOS devs have been pretty harsh towards Fairphone because of their slow updates.

They seem to refuse that they're working with fairphone, so it seems unlikely.

https://www.reddit.com/r/GrapheneOS/comments/1o3vmn5/comment...

My guess is that its either HMD or Nothing. Will probably still take a while until we learn about this