OEM is the new full version of Windows 8
Confusing as ever, Microsoft is releasing Windows 8 in a variety of packages, and resellers are not all making really clear what they are selling. Since MS has realized that the majority of Windows OS distributed are pre-installed in new PC's, the product being offered at retail in the box(es) labeled Windows 8 Pro is an upgrade version that requires an existing copy of XP Vista or 7. As far as I can tell there is no "retail" version of the base Windows 8 Upgrade being released.
The base and pro versions are being released as OEM=Full versions in either 64 or 32 bit, but not both (as was the case with Windows 7). The good news is that the licensing for the OEM version has changed, and now has a "personal use" clause that permits you to move it from one machine you own to another. This is the same kind of capability that the retail Full Version used to have, since that version was never transferable to another owner. Newegg still has the warning that it can only be used for assembly for sale however. These will probably be the versions most home builders will want for their own machines, unless they plan to trash an old license and save some money on a upgrade. Upgrades btw may well not be movable to a new machine once installed.
http://www.zdnet.com/microsoft-radically-overhauls-license-agreements-for-windows-8-7000002866/ For those who bought a machine with the base version installed, the Windows 8 Pro Pack released in retail will upgrade that version to pro, but does not have any function by itself.
Since many online sellers are not making the distinction, some people will undoubtedly buy upgrades thinking they are full versions.
This is what I have found for part numbers--might be wrong so someone with an in at MS should correct them if so.
"Windows 8 Pro" Upgrade Retail 3UR-00001
"Windows 8 Pro Pack" Retail only works to upgrade a preinstalled base version to Pro 5VR-00001
base version OEM/Full 32 bit WN7-00368
base version OEM/Full 64 bit WN7-00404
pro version OEM/Full 32 bit FQC-05920
pro version OEM/Full 64 bit FQC-05956
And of course sane users, and those who want their computers to work for now will simply avoid the upgrade or buy Win 7 while it is still available.