Few months ago I bought an OEM version of Windows 10 Pro for roughly around 1$ off eBay
Everything's fine with it and the key is working, but soon I'm planning to upgrade some parts of my system and probably do a new installation of Windows using Media Creation Tool
I want to know, to what OEM version is tied to (which component inside the PC), and am I able to re-use the same OEM key after re-installing Windows and such
According to the EULA the OEM version is intended for use by System Builders. It is only meant to be installed on a machine that is going to be sold on to a customer after install.
Unlike a full retail key an OEM key is not transferable. It it tied to the first machine it is installed on. Apart from that, it is identical to a retail key as far as activation goes. The digital licence obtained is stored on Microsoft's activation servers and linked to the Hardware ID of the PC. A significant hardware change will loose activation. The main component of the Hardware ID is the motherboard, the hard drive and RAM play no part in the ID.
As long as you don't change the motherboard you should be able to reinstall W10 and it will activate. You do NOT need to give the key again when doing a clean install. When asked for a key click 'I don't have one' then install the same edition you had before (Pro in your case). It will activate automatically from the existing digital licence.
Quote