There is no such thing as OEM office; the correct term, I think, would be Volume Licensed (I believe the correct term is Direct Service Provider)- basically, the manufacturer "buys" the licenses in bulk to preinstall on their computers, which they then sell to the consumer; there are special terms in said license between MS and the manufacturer to allow for this implicit transfer of ownership from the manufacturer to the eventual customer; additionally, these versions, just like the SLIC activated versions of windows that most manufacturers use, come pre-activated.
The confusion here is because OEM & DSP are often used interchangeably- but the software itself is marketed along different channels.
OEM is sold to major computer manufacturers. It is labeled with the manufacturers name (Dell, Gateway), and they supply support for the product. The interesting bit here is that although copies of many MS products are called "OEM" there is no "OEM" label anywhere on the disk itself; these are truly, DSP products.
DSP Direct Service Provider software is provided to smaller computer builders, upgraders, service professionals and consultants. It has no branding, the installer still provides some support to the end user.
The requirements for the different manufacturers for the legal sale of this type of software almost always includes that it be bundled with hardware when sold to the end user.
If you are a hobbyist building and selling computers, or a large company with an IT department DSP software is okay as I understand it.
The dangers are in two areas. First the license is to prevent retail sales of OEM/DSP software to individuals (because it then competes with the manufacturers offerings). The other is fraudulent offering of the software for sale, e.g. A person buys a computer that has preinstalled software, and receives the disks with it. He then tries to sell these OEM disks, when the license has already been used, and the transaction doesnt involve the sale of the computer.
Microsoft software must always have the COA, and the license determines the number of users, who provides what kind of support, and the requirements for a legal transaction.
The bold part I suspect is what may have possibly transpired here.