Once an option group has been allocated to an instance, it cannot be deleted. The option group must be removed from all instances. When attempting to delete an options group with dependents, the following error is returned.
The option group ‘audit-options’ cannot be deleted because it is in use.
To identify to which instance the option group is associated, simply click on the option group. It will show “Associated DB instances and snapshots” along with “Option group properties”.
Go to that instance and change the option group of that instance to remove the dependency. And then try to delete the option group again. In case there is a snapshot, there is no other way to modify the option group for the snapshot except to delete it.