I can't seem to find the concept of using outside oxygen supply for an inside fire whether it be a stove or fireplace. The logic behind this is that you can avoid drawing air that you've heated from the living space to supply the fire but instead supply the fire with plenteous oxygen from outside that you haven't already invested expensive fuel to heat. Are you aware of systems that employ this method of oxygen supply? Is there some danger in such a method?
All fireplaces should use outside combustion air to reduce the amount of pre-conditioned air being used from the habitable space.
Very simply, there should be an outside combustion air supply installed on every fireplace - this will keep the air that's already been heated from being pulled through the system and out of the house.
As a clarification, you're not supply pure oxygen to the fireplace - only a "normal" atmospheric blend.