I verified last week after the Win 10 update that you can enter a Win 7 or 8 key with a fresh Win 10 install, and successfully activate it. I did this with retail versions of 7 and 8 though, so an OEM key may be different.
NOTE: You may have to use the phone verification process. I had one that went through fine, and one that I had to talk to an agent to finish the process.