AFAIK, the OEM discs and Retails discs are the same (RTM) other than the former including drivers/bloatware.
The discs obtained from MS have both Pro and Core - it may be defaulting to Pro (the first index) and that is probably why it's giving the error that the key is not valid.
You could try adding a PID.txt file with the OEM key to the sources folder of the iso and run setup from within Windows 10 - choose to keep nothing, install Win8 and then upgrade to 10.
Save a pid.txt file - open notepad and add your OEM key like this:
This way windows will be forced to install Core.
NB to check your disc - insert it and run this command:
Dism /get-imageinfo /imagefile:E:\sources\install.wim
Change E: to your DVD drive.