Hello,there here is some thing you can check to stop the leaking. It is not likely an O-ring but could very well be if the float is letting the water past the fill line, but here is what I can recommend, I believe this model has a breather or air filter check to make sure these are not wet or blocked. If it is a wet air filter, use a hair dryer and dry it out. Do Not Use the dryer on hot setting warm only...Ok, then replace it and now it should not leak.
If it has an air breather take it out then put the water safe top back in the reservoir and replace the btl. and if it does not leak then it is fixed. Most BTL-
Water stores have the o-rings or can get them in for you if not then you have to order them direct from company.
Some times you get an air lock in the water BTL this also can lead to leaking problems, this is very easy to remedy just do what I call ...burping the the water-cooler, with the water Btl. on the cooler just tip it slowly forward then all
the way back-wards till you hear and see bubbles release to the top of the water level in the Btl. then replace to normal standing position and dispense a glass of water out of cooler, dry area around top where it was wet between Btl and top of water-cooler floor and any where else you see water now it should stop leaking if the air-lock is out of the Btl. Ok let me know if you have any questions. | 
