The OEM License is bundle on your NB/PC (Installed from the factory or you buy OEM License for DOS - I mean NB/PC not include OS when you order it)
For OEM you choose the version 32/64 bit before you buy, so if now you use Windows 32 bit, It's mean you must to buy open license for upgrade to 64 bit.
But if you use Windows open license (They call 'OLP') It's a volume license, you can install any version, just download the version that you want on 'Volume Licensing Service Center', you can upgrade to 64 bit without buying any license