OEMs are full versions of the OS. They do not require a previous version of the OS to already be installed on the Hard Drive. Upgrades, on the other hand, do require a previous version of Windows Installed.
If you have a previous version of Windows available to you, You could install that version, purchase the cheaper upgrade, and then install the upgrade over the previous OS. A lot of work, but if it saves you $40, it might be worth it.