Some nuts are designed with a recessed valley to accomodate an o-ring. This is the style of nut that BargainFittings.net sells. But, on the other side, you do need a washer between the coupler and the o-ring. I agree with Bobby_M that teflon tape is a bandaid. I don't load up the threads with teflon tape.
The problem I see happening often when using fittings like this is that when the system is overtightened, the o-ring pinches under torsion and loses seal. I spray the o-rings down with some food grade silicone lubricant before fitting everything together. That reduces the friction between the o-ring and the washer/kettle wall. I then use a pair of channel locks to hold the coupler still and I tighten only the locknut. Since the locknut has that recessed area, its less susceptible to kinking the o-ring. I've had great success with this procedure on both kettles and coolers. My weldless fittings don't leak one drop.