Park Rangers are not a division of the OHP as Lake Rangers are, so their primary frequencies are probably going to be different..
Per
http://www.dps.state.ok.us/ohp/tngrct/OHPtroops.htm, Troop W is a regular OHP troop.
As you note, the State Park Rangers are organized under the Parks and Tourism. See, for example,
http://www.touroklahoma.com/info/newsreleases.asp?id=848
Q. Do the State Parks have their own law enforcement?
A. Yes. All of our park rangers are fully commissioned peace officers who can enforce all state laws, park rules and regulations. They have the authority to make arrests and carry weapons. The larger parks require a bigger staff, and some of the smaller parks share rangers. Rangers have a variety of duties, from assisting in medical emergencies to keeping the peace in campgrounds, enforcing speed limits and protecting resources within the parks.
In addition to policing their own areas, park rangers often provide aid to county sheriffs departments, other state law enforcement agencies and the Oklahoma Department of Wildlife Conservation.
48784 was long listed as "OLP - Lake Thunderbird", but I see that it's been updated to say Park Rangers; I think this is a better description.
First, there's not a lot of traffic on the TG. Second, all that I've heard seems to me to be Park Rangers, and not water troopers.
I do, occasionally, hear traffic on 156.015, but there's not really enough traffic there to correlate ops there with what's on the State system. And yes, I'm close enough to hear the simplex comms on VHF.