Yes, you can hear all traffic on the dispatch channels (dispatch to car and car to dispatch and car to car).
All of the other talkgroups (channels) such as Impact, tac, and investigations are encrypted and you will not hear them. I see that the tac talkgroups in the database don't show as encrypted, but I have never heard anything on them. You can monitor the dispatch comms on the OKWIN system as well so no Provoice capability is needed for the police, just for the OKC fire dept.
If you want to monitor Bethany, Warr Acres, Mustang or Yukon, Provoice will be required.
However, I have found that a scanner with Provoice gives a much cleaner audio than the feeds to the OKWIN system (Provoice is digital audio). The audio volume can vary quite a bit from one transmission to another, however.
Yes, today's trunking systems are vastly more complex than in the simple days of one frequency per channel. Trunking (similar to the systems that cell phones use) is now used to conserve the number of actual radio frequencies used for a large system. So instead of individual frequencies for each use, a "talkgroup" number is now assigned for each "channel" (such as OCPD Hefner Dispatch) and and the whole thing is computer controlled, with the actual radio frequencies used jumping from one to another. In a very large system like OKWIN, there are many "sites" that supply the actual radio frequencies and computer control for different locations, but the talkgroup assignments are the same throughout the system. Also, the trend is to go to digital audio, too. You will need to read up on this stuff in the Wiki section of Radioreference.