OEM license is tied to the first motherboard it is installed on. When you activate that motherboard specs (hardware hash) and product key are logged by MS. If you then install the OEM W7 on a different motherboard it won't match what has been logged and will fail activation (although I have heard of some that say they have done it)
That is why oem software is cheaper than Retail.
With Retail you can install on one pc then uninstall and get a new pc and install it on that one. (is only on one pc at any one time) (unless you have a family licence which allow 3 different pcs to use it at same time)