poison oak is estimated to be the most common native plant in CA. you will almost certainly see it or pass by it while in lower elevations, although since it is deciduous you may not recognize it if it hasn't re-grown yet (but it can still get you, even when leafless).
i'm no PCT expert, but i live in SoCal and i've heard from more than one former PCT thru-hiker that the best clothing strategy in the southern parts is to wear long pants, as pretty much everything there will hurt you in one way or another - the plants are sharp and the sun is blazing. I always wear long pants in chaparral and poison oak country - just the safest bet.
if you decide to go with shorts but are still worried about poison oak, carry a small bottle of Tecnu.
edit: are you hiking all of the PCT or just part?