It seems strange to do an oil change when working on the AC. Do you know what viscosity the dealer put in? Maybe it's a different one than you have been using, probably thinner, and it's getting by the o-rings. I would do a full change with your usual oil, if you can't find out if the dealer put in thinner viscosity oil.

I hope that no one ran the engine while it was draining, or nearly empty of oil. That would be a bad thing. I hate those commercials that make it seem like an engine can run all day with no oil.