Hello,
I was looking for the same and here is what I found so far.
You can use the following MIB :
- BEGEMOT-PF-MIB
- HOST-RESOURCES-MIB
- IF-MB
Source : https://www.zabbix.com/integrations/opnsense#opnsense_snmp (https://www.zabbix.com/integrations/opnsense#opnsense_snmp)
Here is how I use Zabbix to monitor OPNsense : https://forum.opnsense.org/index.php?topic=33481.msg182023#msg182023 (https://forum.opnsense.org/index.php?topic=33481.msg182023#msg182023)
Be advised that Decisio do not provide an official documentation on how to monitor your OPNsense using an external tool.
Hopes this helps.