If 5 threads is good for explosion proof why is it not good for weather tite?
Because for explosion proof, all the threads do is provide a long enough path to cool the escaping gases below the ignition point of the surrounding atmosphere. They DO NOT have to completely prevent leakage.
Pipe threads generally aren't leakproof without some kind of sealant. Plumbers need to use teflon tape, pipe dope, etc. to keep threaded pipe joints from leaking.
I would use the O-ring unless the instructions say to omit it.