In addition to a robust REST API that allows you to easily get data in and out of OpenNMS, we integrate directly with many other systems and protocols (too many to list here).
These include, but are not limited to:
E-mail, Slack, Mattermost, custom scripts
JIRA, Remedy, TSRM
BMP/BGP monitoring, Junos Telemetry Interface, NetFlow v5/9, sFlow, IPFIX, Cisco NX-OS Telemetry, OpenConfig Telemetry
Time Series Storage