There are two types of OEM licence - One is as stated from the Major OEM Manufacturers, Dell. HP Etc, these licences are non transferable and tied to the Hardware ID of the system allocated, they do not activate with Microsoft but with the Manufacturer, this is automatic based on code in the Hardware or by unique installation media, (recovery disk), also all after sales support and assistance is the responsibility of the manufacturer, Not Microsoft. The normally do come with a re-installation key fixed to the bottom of the case for when a bespoke installer is not available, or the users wants to forgo the gifts included with the original install (AKA Bloatware)
The other type of OEM licence was originally limited for sale only to small independent system builders (it was actually known as a System Builders Licence in some markets). This has almost all the restrictions of the Major OEM Licence but is supplied with a key (sticker) and is activated with Microsoft. As this version was considerably cheaper to purchase that a full retail Licence. It eventually became available to the regular user - Also all support is the responsibility of the small system builder or the purchaser/ user.
I think that most if not all of the legit windows 7 and 8.1 licences now available will be of this second type. If you can find a rare retail product be prepared to pay a premium 3 or 4 times the price of OEM would not surprise me ,