I don't think WD-40 is the stuff to use, as it may make the rubber ring expand until the solvent dries out. Try a spray silicone on the O Ring and fitting.
PERMATEX® Silicone Spray Lubricant
Permatex® Silicone Spray Lubricant, Lubricates and waterproofs all types of mechanical equipment. Stops friction and squeaks; coats hinges and channels. Prevents locks, doors and windows from binding. High silicone formula does not attract dust or dirt.
Suggested Applications:
Lubricates door locks, trunk lids, brake cables and rubber molding.
Prod 80070