I believe when they first went to the PV system OCFD was dispatching in analog, but they stopped soon after. It is all PV now so of course the scanner won't pick it up. As I understand it the radios are (were originally?) set up so on the "mixed" TGs they *can* go analog they just *don't*. Not sure why they would bother to switch, perhaps when the system was new they wanted the option in case they didn't like the PV audio?
The quote you cited from the OKC page only refers to the PD dispatch links. The FD links to the state system aren't kept active 24x7. The FD dispatcher will enable them when and as needed, which isn't too often. Mostly when they are working a fire with someone already on the state system - I've heard that with Nichols HIlls and Midwest City.