The o ring is to keep the gases from passing through to the top of the housing i believe. Yeah, they aint cheap. I think about 5.99 at my dealer PER PIECE. This thread reminded me I needed two for my new 97 XP. I actually found some on ebay, 10 of them for 15 dollars plus free shipping! I woulda paid that for 2 of them + tax + gas expense just to go to my stealership.