Didn't even realize it had an O ring on it. Have one that came with my first detector and had the same issue. Hardly even use it since I upgraded to the carrot but I just used a little electrical tape on it. Batteries seem to last forever in it anyway.
Harbor freight sales a little plastic box full of different sizes either 50 or 100 in there for like $4 we used to use them all the time to fix nail guns.