From OpenNMS

If full automatic discovery is not for you, OpenNMS will generate NewSuspect events for any unknown nodes that send the OpenNMS server an SNMP trap. This means that with OpenNMS running, you can configure your SNMP agent on the host to send traps to the public community on the OpenNMS server, restart the SNMP agent (which typically causes a "Cold Start" trap to be sent), and OpenNMS will discover that node and schedule a capabilities scan.