I've recently upgraded a Windows 8.1 Pro (OEM) to Windows 10 using the Media Creation tool since I was not getting the GWX icon. Activation has been successful from the OEM key. However, there were some registry issues from the Win 8.1 install itself, and these are still present despite the upgrade, and thus I would like to know whether if I format and clean install Windows 10 directly on the PC, will I be able to activate automatically again, or do I need to purchase a Win 10 key?
Once you have done the upgrade to Windows 10 your computer is registered with some sort of digital ID with Microsoft. At that point you can do a clean re-install of Windows 10 and you will be automatically recognized as legitimate.
I worked with Microsoft Windows Technical Support for a little over 4 yrs. I was also part of the launch of Windows 8, 8.1 and 10. What Reverend Jim mentioned is correct, Windows 10, especially those who upgraded from a lower version of windows, are automatically registered using a Hash of your hardware IDs (don't ask how it is computed, I don't know too). When you reinstall using a downloaded ISO burned on a DVD or from a bootable USB, it should automatically register it as a genuine copy.