I just purchased an 07 Camry I-4 and noticed the small O-Ring at the top of the oil dipstick that keeps dirt out of the dipstick was missing. It is very small and the dealer cutaway pictures do not show a part number. I guess it only comes with a new dipstick. After looking several places, I found a perfect fitting O-ring at Lowes.

Danco #36 O-Ring
5/16" O.D. X 3/16" I.D. X1/16"

#96750

Comes in a 10 pack for less than 3 dollars.
Hope it saves someone some time if you are looking for the same tiny part.

 

It doesn't seem to fit my Camry... I'll tell this to my friends.