Both!  You need OMSA installed on all servers, and SNMP configured to communicate with your ITA box.  There are videos that will tell you all about it. Go to support.dell.com/video, look under Servers & Storage, expand System Management, and click the IT Assistant link.  You are looking for the 4-part video, parts 2 and 3 will be most relevant to your question.
Hi I have installed IT Assistance, it has detect the servers but they seem to all be listed in the "unknown" group, do I need to install OMSA on each server or is it the SNMP that is the key here? Thanks dellmonkey  
IT Assistance basics