The only sort of "exception" to this is relating to refurbished computers:
The OEM software is licensed with the computer system on which it was originally installed and is tied to that original machine. OEM licenses are single-use licenses that cannot be installed on more than one computer system even if the original machine is no longer in use. The end user license agreement (EULA) accepted by the customer before they use the software, states that the license may not be shared, transferred to or used concurrently on different computers.
So I hope that clears a little bit of OEM confusion up or it may have caused you even more confusion!
Q. What is the difference between a refurbished PC and a used PC?
A. A refurbished PC is a computer system that has had substantial hardware modifications that may require a new operating system license —because the modifications have essentially created a "new" PC.
Generally, you may upgrade or replace all of the hardware components on your customer's computer and the end user may maintain the license for the original Microsoft OEM operating system software, with the exception of an upgrade or replacement of the motherboard.
An upgrade of the motherboard is considered to result in a "new personal computer" to which Microsoft OEM operating system software cannot be transferred from another computer. If the motherboard is upgraded or replaced for reasons other than a defect, then a new computer has been created and the license of new operating system software is required.
If the motherboard is replaced because it is defective, you do NOT need to acquire a new operating system license for the PC. The replacement motherboard must be the same make/model or the same manufacturer's replacement/equivalent, as defined by that manufacturer's warranty.