As above, although my advice would be to discharge the system (using a professional service only), then get all O-rings replaced while the system is being serviced. When one is worn, likely that the others aren't in great shape so for the sake of a bit more labour, do it all in one hit for peace of mind.
My local aircon specialists don't charge for O-rings when re-gassing, they fit them as part of the service (if any are needed but they also change them if you ask). And they do a discount if you use them more than once, saved me some money when I had multiple failures of O-ring seals on my Mk5 GTI...