When it comes to Windows, “OEM editions” most often refer to the cheaper versions of Windows made available to “system builders” and other classes of Microsoft partners. You can buy OEM versions of Windows online almost any place that sells software, such as NewEgg. Companies keep compliant with Microsoft’s rules by bundling OEM versions with a token piece of hardware, like a cable.
Is an OEM version worth your time? Personally, I would never imagine buying an OEM version specifically because you can only install it on one motherboard and never transfer it to a new computer. That… sucks. There are many other restrictions as well though, so be sure to check out the article before handing over cash for what seems to be a great deal.
Source: Ars Technica