Thanks, I did some reading and found that OKCPD uses the trunked system that is impossible to access with any scanner on the market but they also use many other frequencies around the county to get coverage in those areas so a regular scanner should pick up that stuff. I really don't know what all is possible to hear but using my phone app, which basically just listens to other people's "host" scanners, I've heard OKCPD, OHP, OKCFD and several others so they must still be operating on normal frequencies sometimes.
It's not the frequencies - it's the protocol used to change the voice into digital form. They use frequencies in the same band as other agencies. OCPD is carried on the state trunked radio system and can be heard there with available scanners, same as OHP. You've already figured out the easiest way to hear OCFD.