i recently bought a motherboard and loaned it to a friend to use in his computer while i waited to get the rest of the parts for by build. he installed his own copy of windows 8.1 oem with my mobo and now i am about to get it back from him. i have heard that oem is locked into the mobo or something like that. do i have to use his copy of windows or do i just get another oem copy or should i get the full copy or what?
Correct, an OEM disc is tied to the first motherboard it's installed on, so it can only legally be used with that motherboard.
Furthermore, if you subsequently try to use that OEM disc with a different motherboard, product activation will never succeed.
if your friend is going to give you the hard drive that it's installed on, along with your motherboard, you won't need to re-install.
But if he's keeping the hard drive (though he won't be able to use the OEM Windows that's on it without your motherboard), your friend needs to give you the OEM disc so you can install it on your hard drive (the disc is no longer any use to him anyway now).