this is a very advanced skill and since it is not legal to do on the rings (you must always keep both hands on the rings in competition) very few have done it.
Here is a video of a ts00nami training for it with one arm lever pulls and a 4lbs weight in the non-working hand to serve as a counter-balance:
and here courtesy of Beast Skills are some photos of two others working torwards and executing the skill:
This is John Gill one of the inventors of bouldering and master of bodyweight strength. Notice that he is on an overhang. John says that he was even able to do these levers with a single finger on a similar overhang. For those interested in learning this skill note that the non-working hand can serve as a counterbalance by extending away from the bar. I doubt that anyone has ever done the one arm lever with the non-working arm pressed to their side.
Jasper Benicasa has also said that he was able to do one arm levers, but he is over 80 now and there is unfortunately no photo or film of this feat.
Zef is working on the one arm-lever so maybe there will be a new member of this exclusive club.