Not sure if I follow entirely.. the big difference between retail and OEM (other than packaging) is that you're able to transfer the retail - transfer meaning either to another machine if you replace your PC, or if you decide you want to sell it (both requiring that you stop using it on the original machine - you can only have 1 working copy per license you own).
With OEM, you get the discounted price because you're buying a license that is effectively locked to that particular set of hardware. Retail doesn't have this restriction.
If you are using Parallels, the hardware it emulates - and what Windows runs on - is fixed regardless of what the Mac may be underneath. With the Parallels virtualized hardware never changing (how virtual machines are designed) which is what the Windows licensing is focused on, you'll never have to deal with the transferring that you could only technically do with a retail copy.
Counter this with Bootcamp.. if you run Windows in bootcamp and eventually replace the Mac, the hardware for bootcamp has also correspondingly changed, so by Microsofts terms, you would need to get another copy of Windows.
If you don't know what your long term plans for Windows are (ie: running in Bootcamp may be an option at some point) and you have the cash, get a retail copy of Windows. If you know you're only going to be using Windows inside Parallels, you'll never have to deal with the OEM restrictions, so get the OEM version and save about half the price.
If unsure, get retail. Costs more, but ensures you won't have any issue from MS should you ever need to deal with them.
Either way, you can only run one copy of Windows at a time with only 1 license (regardless of Parallels, Bootcamp, etc).
*lock is a figurative term as you have to call MS after so many activations anyway - it's just an automated system that supplies you with a bunch of #s to enter to bypass online activation. Done it a ton of times.
