I have bought a lot of OEM stuff and I have never had a problem.
Generally speaking if you buy, for example, a MS Intellieye Optical mouse which is not OEM, it arrives in the same fancy box that you would buy over the counter from your local PC store, with all the necessary drivers etc. If you buy the OEM one it might arrive in bubble wrap inside a plain cardboard box - no frills.
The warranty should be the same as non-OEM, but confirm before buying.
The difference is the price. OEM is cheaper.
Hope this helps.
Oh, I'm sorry. Did I accidentally blow your head off.......again?