The DPE for my commercial gave me a list of questions before my checkride. The oral didn't necessarily come from the list but it did help fill in gaps in my knowledge.
Other than that, I recommend (and this works for any oral) simply going through the PTS task by task and making sure you know what it says you're supposed to.
1. Applicant must thoroughly know the aircraft, including electrical system, landing gear system, principle of operation of a constant speed prop, and all performance charts.
2. Weight and balance - longhand method. Work one problem where applicant has to offload baggage or fuel to get the CG within limits.
3. Applicant must know all V speeds.
4. Know the difference between anual, 100-hour, and progressive inspections.
5. What are airworthiness directives? Where are they found? How can you determine if an aircraft has all applicable ADs complied with?
6. What is a service bulletin?
7. What is a Form 337?
8. What preventative maintenance can a pilot do?
9. What are the effects of excessive aft or excessive forward CG?
10. Know aviation weather reports including symbols, area, terminal, and winds aloft forecasts.
11. Pre-flight inspections
12. Aircraft documents (ARROW), including knowing which are permanent and which must be renewed.
13. Know NTSB 830.
14. Medical certificates, including duration for commercial pilot operations.
15. Know limitations and privileges of a commercial pilot (FAR 61.139, 135.1).
16. Know equipment requirements - VOR, transponder, oxygen, etc.
17. Know minimum altitude rules in Part 91.
18. What is hydroplaning? (9 times square root of tire pressure)
19. What is a ground loop? Wheelbarrowing?
20. What is a High Density Traffic airport? What are the procedures to fly into one VFR?
21. What is category? Class? Give examples.
22. What is an ELT? Can an aircraft still be flown with an inoperative ELT? Or an outdated ELT battery?
23. What is wake turbulence? How, where, and when is it generated?
24. Who may act as PIC under Part 135?
25. What are Unicom and Multicom?
26. Know required instruments for VFR flight.
27. Know VASI systems.