Officially and technically from Microsoft - Yes.
From your OEM - Unknown - you will have to ask them.
When you buy an OEM edition of Windows, you are mainly buying the key and not the media.... I buy loads and very rarely ever touch the CD, I usually only stick the sticker on the case and then use my PXE server to install the OS, I then give the media to the client.
The licence says the edition e.g. Windows Vista Ultimate, but it does not say x86 or x64, and the licence is valid on both editions. As like all commodities, the x86 and x64 disks are bought and sold by distributors at different times according to stock levels which means sometimes there are big price differences, I asked Microsoft about this and was told I can just buy the cheapest and use which ever platform I need as long as it is the correct edition.
That being said, I am still only able to give my clients the original disk I bought - if they require a reinstall, I usually come out to them and do it from my own disks... I have no idea if large OEMs will even help you do this.
Again, technically and legally, you can... If the OEM will let you is up to them. This is assuming you are getting an OEM copy from a big company such as Dell, HP etc. If you are going to a small company such as mine who buys their OEM disks from distribution, you can just use any other standard OEM copy and use your key.