The SAP Crystal Reports Server (OEM Edition) is only available to OEM partners whereas the SAP Crystal Server 2011 is for all other customers.
I don't see any real differences between them. You may check the Product Availability Matrix to find out if there are differences in the systems they support.
Here's the PAM for the OEM Edition:
http://scn.sap.com/docs/DOC-20505
-Abhilash