DIAGNOSIS You're dealing with a worn or damaged quick-connect fitting, not just the O-ring. The brass collet inside the female connector gets worn or the retaining balls get gummed up.
FIX 1) Disconnect the wand. 2) Look inside the female quick-connect on the end of the high-pressure hose. You'll see a brass ring and a set of small ball bearings. 3) Use a small pick or compressed air to clean out any dirt or debris jamming the mechanism. 4) If the brass ring is visibly scored or deformed, you need a new quick-connect fitting. They are standard 1/4" size and can be replaced with a wrench.
IF THAT DIDN'T WORK The male end on the spray wand itself could be worn down. Compare it to a new one. If it's scratched or tapered, it won't seal. The wand tip is replaceable.
HEADS UP Never force the connection. It should click together smoothly. Forcing it misaligns the seals. Always depress the collar fully when disconnecting.