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        Using Oxygen is not a substitute for lower altitude. In fact the difference is about 3,000 feet. So even though you are on O's at the South Col (26,300') you still feel like you are at 23,300'! Another benefit is that your fingers and toes stay slightly warmer.
        There are a couple of systems out there. One from Summit Oxygen and the traditional approach using the Russian Poisk System. Both use lightweight metal bottles but they differ in delivery approach. The Poisk System uses bottles filled at the factory in St.Petersburg, Russia. The oxygen is delivered through a regulator to a tube to a face mask that provides a constant flow of oxygen. They hold about 1000 liters of oxygen and weigh about 7 lbs.. Climbers usually run their flow at 2 liters per minute and count on getting about 7 hours out a bottle. The Summit Oxygen approach uses nozzles inserted into the nostrils instead of a full mask. This "on-demand" approach is more efficient but has not been widely used on Everest. Their 3-litre system weighs about 7 lbs. when full. Running at 2-litres/minute it will support about 22 hours of climbing thus require significantly fewer bottle and less weight. Several teams are using it this year.
        On summit night, climbers put one or two bottles (using Poisk) in their packs. The rubber tube runs over their shoulder to the mask. Many climbers put tape on their cheekbones and nose bridge since the masks will rub those areas raw after 18 hours on a summit climb. The mask will interfere with visibility in that it is almost impossible to see your feet. This makes you go even slower! And since the mask is not 100% sealed around your face, air will escape and fog up goggles and sunglasses. All in all it is a fairly uncomfortable situation. But very few climbers would trade the discomforts for that extra 3000 feet!