Microsoft does not fully permit you to virtualize OEM licenses. OEM licenses should stay on the same hardware that you purchased them, it's non-transferable and that is why OEM licenses cannot be used on live migration and HA.
Instead better buy win 7 enterprise (not pro) licenses. There are some programs by microsoft that offers discounts like open licensing for small-medium enterprise.
For more info on license, better look at this link:
http://www.microsoft.com/licensing/about-licensing/product-licensing-faq.aspx#tab=2
Regards, Harold 