The resin from poison oak contains the toxin and is not broken down by normal soaps. I know there are soaps (bar type) out there that will break down the resin and allow you to wash it off. We buy them at a health/whole foods place. I would recommend going to or calling one of these places and asking them. There should be a liquid form available I would think that you could use.
Meanwhile I'm going to do a search to see if I can find the name of the stuff we use.
ETA: Here's the stuff I use http://theoutdoorworld.com/products2.cfm?id=48404