No difference as far as I know mate. I got an eom 2600k and it OC's just fine. I'm only using 1.18v for 4.0GHz. I need to get a better cooler if I want to push it further.
The only difference between retail and OEM is that the retail one comes with the Intel stock cooler and the OEM one comes in a smaller box with no cooler, they are still the same CPU though.