PP,
The "oral" and "checkride" are the same thing, and are called the Practical Test. You must pass a practical test to add category or class to your pilot certificate.
You do not need to take another knolwedge exam, still often referred to as a "written" test.
The scope of material covered by the examiner for your single engine privileges will be less than covered for the multi engine practical test. Many tasks have already been demonstrated, and you do not need to repeat them. The same goes for the oral portion of your practical test. The examiner may ask you anything covered by the practical test standards, and may reasonably expect you to be able to answer and perform to commercial pilot standards (even if it's been asked or demonstrated before). The short answer is be prepared.
