I am, thank goodness, not sensitive to poison oak, but my wife is very sensitive. She has had good luck with a soap product called Technu. It, too, is expensive, but it's worth it if it stops a bad rash that lasts for a couple of weeks.

Also, any time she has spent time in an area where she knows or suspects the presence of poison oak, she takes off and segregates all the clothes she wore, then washes them at once in hot water. She takes a thorough, soapy shower as soon as she has disrobed. This strategy has worked well for her.