Make sure you're looking at the correct definition of "OEM".
For example, when I built my PC, I bought the "OEM" disc from Micro Center...however, it was not tied into the motherboard (as I was not buying a prebuilt model), so it was actually a "retail" copy of Windows.
The real difference between OEM & retail is:
-- OEM comes preinstalled on your prebuilt PC (Dell, Gateway, HP, etc.);
-- If it actually comes on a physical disc, the disc is usually labeled "Recovery Disc", & can basically be used to wipe your hard drive clean & install Windows from scratch (that was my Windows XP disc from my old eMachines PC)'
-- If it doesn't come with a physical disc, there's usually a "Recovery Partition" on your hard drive, which is used just like a Recovery Disc (i.e. in emergencies you can wipe it clean & start from scratch)...or, if you're not so lucky, it prompts you to create "backup" discs (essentially homemade recovery discs), like my mother-in-law's older HP laptop.
In all 3 cases, the OEM copy is literally tied to your machine, as it only comes with the hardware drivers for the hardware your PC came with. Upgrading or replacing some of the hardware is possible -- GPU, CPU, RAM, additional hard drives, etc. -- but replacing the motherboard or primary hard drive is where you need to buy the new version...usually because you don't have the right drivers (new motherboard) or because you no longer have the Recovery partition (new hard drive).
Retail copies, OTOH, whether labeled "retail" or "OEM", are usually on physical media (DVD or USB) -- although a digital download from Microsoft is possible (usually so that you can create a physical media, though). Although still limited to "one license, one PC", they come with the full drivers from Microsoft, so you can always transfer the license to a new PC as needed (provided the old PC is no longer in use).
So... to your original question...whether the vendor calls it "OEM" or "retail", make sure it's the one that you can buy as a stand-alone. Don't get the "this disc comes free with Motherboard X" one.