What's the level in the tank now?
I'm going to suggest you STILL replace that O ring, if you have one that looks weird. You may or may not have overfilled your tank, but when it comes to tank 0 rings and tolerances, it's possible your "weird" 0-ring may be maintaining PARTIAL tolerance as far as filling, meaning it might hold a vacuum to a certain POINT, say 1/2 or even 2/3 full, but can't handle a total fill (90% full). As it has vaped and leaked, it's possible it's just reached that "point" of fullness it can handle.
I know it can be annoying to strip down and replace 0-rings and whatnot, but intermittent leaking can be a sign the tank isn't quite "right" so to speak. Personally l would remove, replace and fill the thing (not on the mod) with a new 0-ring and see if it performs better. Alternately, I guess you could remove the tank from the mod, fill it to full, and see what happens. If it leaks, you will have your answer (at the expense of some eliquid, possibly) but you do want to make sure this problem is actually "solved."
If my coil is inserted correctly and my build (if it's an RTA) is good, the only reason I've ever gotten for leaking would be a tank that wasn't well machined and needed to be replaced, or O-ring issues.
Best of luck,
Anna