The OEM thing means you haven't really got an official version of windows.
The licence wont cover you. It's for manufacturers to install on a PC - so they will buy 100 licenses, install them on PCs then sell the PCs. In many cases you wont even get a disc - just a serial number and a sticker.
Whether that matters to you or not is up to you. You wont be hassled by pop-ups, etc, like with fake versions, but there is a chance you'll be on your own if you need any support from microsoft that lets them dig deeper into your system.
There is also a chance that you cannot upgrade the OS in the future and a chance that you'll never be able to install this version on another PC in the future (it's normally locked to the motherboard when installed by the manufacturer). Even then, sometimes people call MS and say they had to change motherboards because of a failure, and they allow it. But it's still a risk as with all of this you are really side-stepping a real licence.