OEM (Original Equipment Manufacturer) means that the product comes without bundled software and original package, actually meant to be used in computers sold by the company. The company you buy it from is responsible for any warranty instead of the manufacturer (ie if you buy an Intel CPU OEM, Intel won't give any support or warranty).
So, some components are probably better to risk buying as OEM than others. For instance, a CPU doesn't come with bundled software, and installing it is more of a mobo question as far as support is concerned. If I can find an OEM CPU with a decent warranty, it may be worth the risk, eh?