This is a multi-step process. I'll try to point you to the appropriate parts of the documentation.
I'd start with the Assure User Guide, under "Modifying default global policies to control performance". (Yes, that's unintuitive.) Enable a "Maximum Storage Pool Used Capacity" threshold. In OCI, NetApp aggregates show up as Storage Pools. This threshold should enable you to see Storage Pool Capacity violations.
Then look at "Violation Severity Settings" in the same manual. Set the severity for Storage Pool Capacity to a relatively high level.
Then look at "Setting Violation Notifications". Enable sending of the desired form of alert for the level of violation you configured in the prior step.