You may want to look at the Oral guide and analyze what the differences really are between the Commercial and the Private. (Hint: Not much. But you'll want to know the limitations of a Commercial certificate and where those limitations come from in the FARs... Which will lead directly to what needs to be studied if you remember everything from the Private rating...)
My oral wasn't heavy on Private knowledge but the examiner set up a very long XC to assess that. If that had been done wrong, they'd know what to harp on. After that it went into specifics of legal limitations of a Commercial certificate and then heavily into aircraft systems (you're a Commercial applicant, expect to be able to explain every system on the aircraft in detail, including stuff like what voltage the voltage regulator drops out at both low and high -- if it's in the aircraft manual, it's fair game)...
Also airworthiness and determining it is a big deal at the Commerical. Know exactly the rules for minimum equipment lists (the scenario is, of course, that you've been assigned to fly an airplane and something is inoperative -- can you legally fly it? -- since that's a common place for a new Commercial pilot to find themselves, flying poorly maintained junk...) and also what things can change an MEL or what to do when an MEL doesn't exist.
Know exactly what "holding out" means and why it's a hot button item for FAA these days. Expect the standard scenario, "Your friend wants to go to Vegas this weekend. Can he pay you to fly him and his friends there?"
Etc.