yes i personally installed a OEM version of windows 7 ultimate a week ago. After that i activated it and did all the windows update's, now i have the option to upgrade it to win10.
OEM discs are only meant to be shipped with prebuilt PC's. The discs are not meant to be sold. You might have a problem with the Windows 7 key, which would in turn affect the upgradebility.
what mmaatt747 said is true, i had to use the microsoft phone service.
you basicly navigate trough a few phone menu's, type in the code you get into your pc and you are activated.