OEM stands for "Original Equipment Manufacturer", which means that, in a perfect world, it would be a genuine Samsung part.
But in our world, you often have Chinese companies wrap their 50 cent battery in a copy of a Samsung sticker and sell it on eBay for much more.
The only guaranteed way to get a genuine Samsung battery is to buy it from Samsung. If that's too rich for your blood, then you have to accept the risk of getting a knock-off.
I got a battery for my NS4G from Samsung with a 50% off coupon code for registering the phone with Samsung, if you haven't previously registered your phone you might be able to get the same deal.