First, clean it with a mild antibacterial soap, rinse it well and pat it dry. Do that two or three times a day and see if there's any change. If it doesn't appear to be any better, call your vet. He/she may tell you to try some Benadryl for a day or two.
I'm assuming that at this time of year in Oklahoma the poison ivy and poison oak has died back to the roots, although the irritant oils will still linger for a long time on the dead leaves and vines. That is one possibility, though.
Check around the places your dog likes to sit and see what's there. It may be just a contact reaction to something. It will give you a better idea of what to ask the vet, though. The more information you've got, even if it's just ruling out possibilities, the easier and more likely it is that your vet will be able to get the right answer.