I’m as allergic to poison oak as anyone I know; I’ve “gotten” it even from contact once removed, from the coat of a dog, someone else’s hand or jacket. The swelling and itching is very bad: eyes swell shut, skin won’t tolerate clothing. I’ve found two things that work better for me than anything else.

For the itching, take antihistamines. These work also for relieving the itching of mosquito bites.

For the swelling, and to make the poison oak lesions dry up and go away faster, try Neutrogena’s Body Clear Body Scrub. Put it on the affected areas generously and gently (do not scrub) and let it sit for 3 or 4 minutes before rinsing. One application usually is enough; if not repeat after a few hours. The active ingredient is salicylic acid, 2%, meant to dry up oily skin, but for me it has been extremely effective at reducing swelling and itching, stopping “weeping” from the lesions, and making them heal in perhaps half the usual time. I expect that other preparations with similar content of salicylic acid would work too, as long as no other ingredient irritates the poison oak blisters, but this is the only one I have tried. I keep a bottle just for poison oak season, year after year.

The active ingredient that causes the allergy is urushiol, and as far as I know it is the same in all three species of poison oak and ivy, so this remedy should help in all cases. The fact that the condition is an allergy, not a case of “poisoning,” explains why reactions vary among individuals and even in the same individual. A person previously non-reactive can develop the allergy after any number of exposures.

Disclaimer: I’m not a doctor, nor do I have any financial interest in Neutrogena.

Images below from Wikipedia: left, glossy new spring growth of Western or Pacific poison oak (Toxicodendron diversilobum, also called Rhus diversiloba. Brand-new spring foliage is sometimes crimson.
Right, the attractive fall colors.