Two things may happen, it will work fine (not common) or you wil have 3 days to activate (more common).
Go through the steps presented to re-activate your OEM copy. It will take a phone call to MS via robo-phone, that is a recording.
You should be presented with a long series of numbers, they will read back a long series to enter and you're good to go. This process will re-register the OEM Windows to the new hardware.
You could activate with the key on the new copy of Windows I think too, I haven't tried it with an OEM version, but you'd wastes a good key which you could use for another PC. The OEM and your purchased copy will have to be the same version, Home, Ultimate, etc.