You'll get the message about "registering within 30 days", and it will work during that time. However, at the end of 30 days and you can't provide a valid key, it will stop and you'll have to reinstall all over again.
Microsoft intended that an OEM copy of Windows was "tied to the 1st mobo it was registered on". Therefore (1) you shouldn't be able to use the same OEM copy on 2 computers, and (2) even if you only replaced the mobo on your one computer, you'd need to buy a *new* OEM copy of Windows.
I don't know how well they police this, but it would be very easy to do through Windows Update.