If I am prepooing (LOL) with an oil I do so on dry hair. But if its with a thick conditioner like Aubrey Organics GPB then I do so on wet hair. Its difficult for thick substances to soak in my thick hair when its dry.
Definitely on dry hair. I always prepoo with oils-lately just extra virgin olive oil.
I get the best results with that one oil. I know coconut oil is supposed to penetrate the hair better, but evoo is so much better IMO. I actually see results.