I hunt the trinity riverbottom alot and got into some p. oak 2 years ago very bad. The only thing OTC that I have found is rubbing alcohol and mineral spirits, applied to the site, and then washed off, seems to have a mitigating effect. (first alcohol then the spirits). That has worked for me if I could catch it w/in a few hours of getting into it.
If you are a few days out, then there is some white or clear cream you can get by prescription that helped me somewhat (fluocinide or something like that). But as you already know, after it sets in....its really just a hellish
experience. Good luck.
I have also used Technu, which is mineral spirits + other stuff. It can be helpful if you catch the oak in time. But, again, once you are already infected I think you need to get the prescription stuff and ready yourself for 2 weeks of treachery.