I had a drippy MS660 and it turned out to be the oil tank vent that was leaking. The vent has a small valve that is supposed to let air in when the tank empties but the valve is supposed to close when the saw is not running and drawing oil from the tank. I took the bar and the little guide plate off and I cleaned everything up and let it sit overnight. This is what I saw the next morning - oil was flowing out of the little aluminum colored vent valve that is located on the backside of the bar:
I dumped the oil out of the tank, sloshed some gasoline in to clean things out, then blew compressed air in through the side of the vent that faces the bar. I could hear the air rushing into the oil tank and did this a few times. I can't remember if I blew any carb cleaner, brake cleaner or gas into the vent to help clean any debris out. I then put it all back together and it has not dripped any since.
This may be a problem on the 260 saws as well. When the oil tank pulls air in the tank a small piece of dust can get stuck in the vent valve.