|I have been highly susceptible to poison oak for about 30 years, and have had it numerous times.
This is my experience with hot water.
Not only does a hot shower feel extremely good on a patch of poison oak, hot water opens up the pores and lets the toxic oils out. (Cold water does not allow this) I personally turn the water on as hot as I can stand it, and let it pelt the rash until I can no longer stand it. It relieves the itchiness, as well as dries the infected area, and gets rid of the oils. (as the oils/toxins are what cause it to spread) Be sure to use soap and wash any skin around the area to make sure it doesn't spread. But this method dries it up fast, so it will shorten the time it is spreadable.
I am in no way a doctor, but no doctor has ever cautioned me against this.