I think part of the confusion is because of how "OEM" is defined.
In Microsoft's eyes, an "OEM" copy of Windows is the machine-specific copy that comes with a prebuilt PC from a main vendor (HP, Dell, Gateway, etc.), where either the physical disc or the Recovery Partition of the drive is a vendor-specific version with drivers linked to that particular system...& especially the motherboard it has. That's why a) the disks are labeled "Recovery Disks", & b) why you can't just pop your old eMachines XP Recovery Disk into a wiped HP desktop & expect it to be able to install correctly.
In retailer's eyes -- Micro Center, Best Buy, Staples, etc. -- an "OEM" copy is the "vanilla" version of Windows. When you install it, it has all of the generic drivers to allow you to start the install & initialization process, which you then supplement with the driver installation disks that come with your motherboard & other hardware. The confusion is because Microsoft calls this the "retail" version, indicating that it's sold separately from an actual system.
In either case, unless you spend extra for additional site licenses (something which I think only applies to Enterprise versions or business customers), it's a case of "1 license, 1 PC". The only difference is that the true OEM copies can only be installed on the original motherboard, while the "retail/OEM" models can be reinstalled on a new board provided your old system is no longer in use.