I assume you're referring to making your OEM infrastructure work with Exadata. (The actual OEM OMS should be hosted separately from the systems being monitored, which would mean outside the Exadata rack.) The quickest way to get agents etc set up would be to use the OEM automation kit in MOS note 1440951.1. The download page at Oracle Enterprise Manager Grid Control Exadata Monitoring plug-in bundle</title><meta name="Title" co… also has links to the documentation for each plugin.
Oracle also has a whitepaper on Exadata monitoring with OEM, though it comes off as more of a sales document: http://www.oracle.com/technetwork/oem/exa-mgmt/exadata-mgmt-twp-1662642.pdf