OEM means the software won't be supported by Microsoft, most pre builds you buy from other providers would support the software they put on it.
In your case if your looking to buy retail package vs OEM.
you'd be able to call MS if you needed help with your Retail version of windows.
If you bought OEM you'd have to do the support yourself if you built your own PC or contact the people you bought the PC from, IE bestbuy, Ibuypower, Dell etc.....