> Observers (like a NOC)
> Administrators (People who run the server/service)
Yes, I think that's right, and I can't think of a significant likely audience for the docs that isn't listed (if others can, please sing out!)
Most users are network administrators or other folks in the IT department pressed into that role (in other words, experience levels vary, but most of the OpenNMS users I've met and talked with so far were senior, savvy people.)
NOC/operations staff would be somewhere between observers and users; in the FCAPS model, OpenNMS is routinely used to do F(ault management), A(ccounting) and P(performance); in ITIL-speak, it's Event, Incident and Problem management, plus parts of Availability, Capacity, and Performance Management.