Skyking,
The examiner may ask of you anything contained in the practical test standards for commercial pilot, single engine land. The same applies to the oral portion of the exam.
However, it is most likely you will be asked to demonstrate some performance maneuvers and routine maneuvers at the commercial level, with little else. You will probably shoot a couple of approaches, and perform some air work to include a chandelle or lazy eight. If these have been demonstrated before, you may not get asked to perform them, but then again, you may.
Your question appears to ask if you must pass the practical test for the SEL class rating. Yes, you must. The nature of the practical test involves both flying and talking, so the oral is part of the practical.
What will actually be performed on the practical test is up to the examiner, but it must include enough representative tasks from the practical test standards that the examiner may be satisfied that you can operate the airplane at the commercial level. You should contact the specific examiner with whom you intend to fly, and see if they have any input on the ride they will give. Many examiners can give you a rundown on what to expect, with them.
You will also require an endorsement in your logbook by a flight instructor, recommending you to take the practical test. You actually require two endorsements; one attesting to your competency in knowledge for the rating sought, and one attesting to your proficiency (flight) for the rating sought. This provides that you must have received training or instruction from the person signing your logbook (in order for them to have found you proficient).
No knowledge exam ("written") is required.
When you get this endorsement, get it from an instructor familiar with the examiner to be used. He or she should be familiar with what that examiner typically does on a checkride.
