Use OC server to do the monitoring and in storage system add OC server as SNMP server. Doing this way will make sure that your OC is pooling the filers at periodic interval to get current status and at the same time if anything goes wrong at filer it will send the SNMP trap to OC server notifying the current status.
Say if you have made volume full scan in OC server to run every 10 min. and last run was at 11:30 but a volume went full at 11:35 so until next pooling interval which is 11:40 you will not come to know but having SNMP server configured on filer, filer will send a SNMP trap to OC server at 11:35 itself and OC server will do pooling of that volume at 11:35 only.
so I would suggest use both the methods,