You could be right about o-ring depending were the leak is coming from. The reservoir fix is common problem with the s80. I would have another mechanic look at it they should come to the same conclusion as the dealer if this is the problem. If you cant find a independent Volvo guy then I am sure a European mechanic can handle it.Weither you get it fixed or not a second opinion is a good idea. If you need further help let me know.