OEI (original equipment installation) - is used by system builders and is the same as OEM in that the licence is technically tied to the system (motherboard). However, you can blag MS, especially if you ring in the evening, and get it moved to a different system (no guarantee but I've always managed it) if you're polite and talk like you know what your doing (helps if you do). And, yes, you can get them from a forest but be sure that they're despatched from the forest and don't have the words - 'no returns once purchased'
Retail gets tied to your MS account and can move from system to system. There was concern that MS would meddle with this with the release of Windows 10 as they were so cagey about the licence details with the free upgrade - but they seem to be allowing system swaps with no issues when licences are tied to an MS account.
Either way - you can get Windows 10 very cheap, and relatively safely, if you look around - including your Paul vid.