I don't think there's any major difference in terms of upgrading Windows for OEM vs. non-OEM editions. They all have specific license keys, and, AFAIK, that's the only thing the Windows 8 installer will look for.
I've upgraded the OEM version of XP to Windows 7 (with the cheaper Upgrade edition) before without any problem, and unless Microsoft's said otherwise I would think that it would be the same way for Windows 8.
If you read the "4. What is RTM, GA, Retail, and OEM?" section here, it seems that the only major difference between them is that the licensing key is stored in and tied to the BIOS, which shouldn't be a problem for the Windows 8 installer, which (as far as I know) just checks to make sure that the current version of Windows is activated with a valid license key.
Also, at your source link, it says:
if your PC is running Windows XP, Windows Vista, or Windows 7 you will
qualify to download an upgrade to Windows 8 Pro for just $39.99 in 131
Windows XP, Vista, and 7 are all available in OEM versions, and since OEM versions make up the majority of Windows sales, I think Microsoft would've found a few words to say that OEM versions weren't eligible for the upgrade if that's what they were planning :)
TL;DR: Microsoft doesn't make distinctions between OEM Windows editions and Retail Windows editions.