I am horrifically allergic to poison oak. I am also an avid mountain biker in northern California. This not a good combination. I have tried all the soaps and wipes, but none of them really help, especially if you are out all day and can’t get to a shower soon after exposure.
I have found only one reasonable way to stop the oils from getting into my skin: Invisible Glove, a skin protectant used by mechanics so they can wash their hands clean after working on cars. There are similar products sold specifically for poison oak, but they cost vastly more, and work the same.
I find that if I rub my legs and arms down with Invisible Glove and shower with a soap like Dawn or Tecnu right after riding, I either don’t get any rash, or it’s much more limited and tolerable. It is still worth being as careful as possible, and making sure that clothes get right into the washer (to prevent poison oak oils from getting all over the house), but Invisible Glove seems to be the best preventative for weeks of awful rash that I have found.