A computer that comes with an OEM license has an activation key that is tied to that particular hardware. It is typically not legal to install the OEM operating system on a different machine.
Sysprep was not designed to work with OEM licenses, but only volume licenses.
If you plan to use a large number of machines with OEM licenses, it would be simpler to take one model, format the hard disk and install a 'volume license' version of the OS. That way you can clone it. Note: You will essentially lose the OEM license. You will also be using additional instances of your volume license. For licensing questions, contact your reseller or Microsoft.