How early in May? We started June 11th and didn't have much cold weather, but I think we picked a particularly warm year. May can be kind of early, places like the McKenzie pass may still be snowed in (you can take Santiam Pass if it is).
The coldest days I was OK in tights and a jersey, sweater, and shell on top. Some kind of a cap that will fit under your helmet is a good idea. As is a pair of gloves with long fingers. We did freeze on the top of one pass, but rode to lower warmer altitude to camp.
I wore lightweight zip off leg pants over tights on cooler evenings in camp.
We didn't need real heavy clothes. Just layer up the lighter clothes you take and as was already said forget the cotton.
We didn't bother with booties, and I never wore rain pants on the bike (I did in camp). I took a very lightweight stuffable rain jacket (no hood) and pants, cheap coated nylon ones. They worked out fine.
I have a list of what we carried in my journal at:http://www.crazyguyonabike.com/
This message was edited by staehpj1 on 10-14-08 @ 9:46 AM