An oxygen concentrator. These contain zeolite. When pressurized room air is c over the zeolite, the zeolite absorbs almost all of the nitrogen in the air. The oxygen is directed to a tube to the patient. After each cycle, the pressure on the zeolite is released, which similarly releases the nitrogen, which is sent out an exhaust. This is a lot more energy efficient than electrolysis, doesn’t require the water to be recharged, and works as long as you have electricity and air.
By the way, electrolysis is horrendously slow and energy hungry. Do you know how much energy hydrogen and oxygen give off when they combine? You have to put in even more energy to get the oxygen and hydrogen back out. If you ran room current through a bathtub full of water, it would literally take years to turn it to hydrogen and oxygen.