|I've been walking my dog nearly every day since about March in an area full of poison oak. Even though I am very allergic, I hadn't been affected by it until now (October) and remembered that I have spent nearly every Halloween or Thanksgiving for the last several years with a poison oak rash.
I realized that I got it this time a few days after our first rain of the season. Plants were looking very green and healthy and glad to have gotten a first drink. The dog was feeling giddy and bounded up a hill after some wild turkeys. Now I'm reading this site desperately searching for a miracle cure to have been discovered since my last bout with the dreaded weed last Autumn.
My conclusion -- Poison Oak (in California) gets more potent with new growth after the seasonal rains begin in the Fall.