The OEM version is the full version, but it is meant really for system builders, as it comes with no instructions, etc (wouldn't bother most. I'm running Windows 7 Professional OEM). It's exactly the same when you install it except for a few things:
The OEM versions lock to the motherboard installed. Even though you can install new hardware to a certain extent, changing your motherboard makes the OEM version you have useless. It simply won't boot, whereas I believe retail versions have a few hardware changes allowed from Microsoft.
The retail versions I believe also allow running on 3 PC's but the OEM only one.
No direct support from Microsoft (no biggie)
OEM versions cannot be used as updates, to update an older to a newer.
Have you tried Windows 8.1? No likey?