So at delltechcenter.com/ome there are a bunch of whitepapers. If you go to the Setup section, there is one called Making My Environment Manageable... and under Discovery there is one as well.
So folks often don't realize that their managed nodes are in a wide variety of different states even though they think they are all pretty much the same 🙂
So find a single Linux box with OMSA, discover it with SNMP and confirm that it shows up under the Servers group. Then for the others, you often have to do a "what is different" exercise.
First step is to use the Dell Troubleshooting tool on the OME desktop and point the SNMP test at one of the boxes that is going under unknown. Do the SNMP test results include the version of Server Administrator in the result set? It should if it is working properly. If it doesn't, check the FAQ or the First Time setup tutorial in the OME tutorial section for special Linux managed nodes settings re SNMP.
There may still be an old Linux setup video on delltechcenter.com/ome at the bottom as well.
That should get things rolling.