The possible errors will generally depend on how your application is written to handle errors thrown by Oracle. You are definitely going to want to test any functionality that interacts with the database.
Who will have to do a system testing to check the all the functionalities are working.
1> if your system has any file uploads then may be permission related issues might occur to execute the functionality.
2> If the file uplaods are done using any DTS package then testing of file upload will require.
3> Do check for performance as I had encoutered couple of issues and had got fixed.