Hi Gerard,
There is no SAP note which mentions to test business functionality post Oracle upgrade.
But it is good to perform basic testing of basis transaction and activities like backup, job scheduling.
Incase there remote DB links configured on oracle database, then this functionality should be tested.
From my experience, I have seen a customer who has developed some custom program which execute native sql commands based on certain oracle release.
So post oracle upgrade this functionality was required to be re-developed to match with new oracle release.
So check out with your business users whether any such functionality exists which may get impacted with oracle upgrade. Include those into your testing scope.
Hope this helps.
Regards,
Deepak Kori