It will not activate online. You will have to phone MS to activate by talking to a human. What happens depends on the mood the support guy is in.
What has happened to 1 guy I know is that the guy simply said, "No. We can't activate it. An OEM copy is permanently tied to the motherboard. Have a nice day."
However, I've also heard that if you call and say, "Hi. This is Bob Doe from BD computers. I'm replacing a motherboard under warranty for a customer. Would you be able to reactivate the OEM copy of Windows for the new motherboard?", the answer tends to be "yes".