As person above said, you can only have it activated on 1 PC at a time. If you activate it on the New PC it will become inactivated on the old one. YOu don't have to do anything on old one, just follow the link I showed and activate the new PC with your email address.
OEM is the version of windows that comes installed on a laptop, The other form is retail which is sold separately from a PC and about only difference between the two now is you can probably install Retail on more than 2 PC in its life time. Prior to a recent version of WIn 10, you couldn't easily move an OEM copy of win 10 from one PC to another but its much easier now.