Nagios Configuration Woes

I am rather new to using an NMS suite such as Nagios or OpenNMS. I have used Paessler before but only for the time I had the unlimited censors for 30 days. I have configured Nagios and have it up and running. The problem is, it seems like a pain in the ass process to add the OIDs I want to monitor on each device. Paessler was nice because it would just discover them "automagically". I get that they're both very different software suites but is there any way of making Nagios a little more efficient when it comes to getting set up quickly with scanning OIDs?

EDIT: is there anything that works like PRTG but doesn't have the astronomical cost?

UPDATE: Got Nagios Core and OMD/Check_MK to work. Trying to configure SNMPv3 now.