I read the answer in How much oxygen do commercial airliners carry? but it doesn't go into any details about two things:
How much oxygen is carried? I do imagine it would be quite a bit.
How much weight does it have? I am imagining that it probably would be liquid oxygen as that it would be much easier to carry, more compressible, fill everything better than in the gaseous state. I have been reading about liquid oxygen in different contexts hence thought it would be good to ask.
An A320 carries oxygen generators for passengers and cabin crew, pressurized oxygen for flight crew (including observers) and some portable oxygen bottles [1].
I can't find the exact volume of the pressurized container, but it is pressurized to around 1800 psi (124 bar) when full; [2] (page 148) gives minimum pressure for flight and how long for it should suffice.
Average amount of air inhaled by an adult human is usually given around 2 000 l/h, which contains around 400 l of oxygen (at standard pressure). The passenger masks are required to last quarter an hour, so they need to provide oxygen that would have 100 l at standard pressure (but at 0.15–0.2 bar only) per passenger. Plus there are some (around quarter) extra masks for people who are walking in the aisle at the moment decompression occurs.
Only flight crew masks must be able to provide pure oxygen at sea level pressure to be able to keep smoke or fumes out in case those enter the cockpit. These are always supplied from a pressurized bottle, because it must be possible to turn them on and off as they may be used in other cases too (fumes, pilot feeling unwell, preemptively if the pilot is alone for some time etc.) and the chemical generator will release all their oxygen once triggered.
The answer to your question is that it depends on the aircraft, the flight operation, and what you consider onboard oxygen.
MD88Fan and Alan Hudec have pretty well answered your question. Although I think that you have a slight misunderstanding about the oxygen carried aboard any aircraft. To better understand, let us call the oxygen “Supplemental Oxygen”.
The percentage of oxygen in the air is fairly constant regardless of the useable altitude of most aircraft. At higher altitudes, there is just less air. Less air means that there is less oxygen at the same percentage of oxygen in the air. Just as important is the lower pressure of the air. The lower the air pressure, the harder it is for your body to infuse your blood with oxygen.
At higher altitudes, the only ways to keep your blood oxygenated is to increase the percentage of oxygen reaching your lungs, or to pressurize the air reaching your lungs. Supplemental oxygen can do one or both of these, depending on the supplemental oxygen system. As altitude increases even more, it becomes less possible for the body to infuse even 100% pure oxygen due to the lack of air pressure. That is why a continuous supply of supplemental oxygen can be used in non-pressurized cabins up to a certain altitude.
In a pressurized cabin like the one used for the Airbus family of aircraft, the air in the entire cabin is kept at outside ambient air pressure at and below 8,000 feet MSL Pressure Altitude. Above 8,000 feet MSL Pressure Altitude, the aircraft engine compressors will supply air pressure to keep the inside of the aircraft at 8,000 feet Cabin Pressure Altitude. If the system were to fail, supplemental oxygen is supplied only to the passengers and crew, and not to the entire inside of the cabin.
In a failure of the pressurized cabin and or reduction of cabin pressure, the air crew has pressurized and sealed oxygen masks. The cabin is supplied with portable pressurized masks. The passengers are is normally supplied with chemical oxygen generators that should last about 15 minutes. The oxygen supplied is of a high concentration. But, it is at a minimal increased pressure.
This gives the air crew plenty of time to perform an emergency descent down to 10,000 feet MSL or below, where the outside air pressure is enough to make the air inside the cabin breathable. For comparison, Denver, Colorado has an elevation of over 5,000 feet MSL and Vail, Colorado has an elevation of over 8,000 feet MSL.
The only normal exception to the above are aircraft that have flight paths that require extensive time over very high terrain. Aircraft flying in the Himalayas are an example of this. These aircraft will be outfitted with pressurized oxygen masks supplied by pressurized bottles of oxygen for each occupant on board. These bottles take up useable space and payload capacity. But, they are necessary to provide each occupant both the oxygen, the pressure, and the time necessary for the aircrew to find a safe path down to a lower altitude.
It's VERY rare for an airliner to carry actual oxygen. Most carry chemical oxygen generators that use an exothermic reaction to produce oxygen.
This video explains in detail how the system works. Overly simplified, when you pull down on the mask (and you should pull with some force) you're pulling a pin to start the reaction that generates oxygen for your row of seats on whichever side of the aisle you're on. So if you're sitting in the window seat of a 737 and you pull your mask, you start the O2 generator for the 2 people next to you. However, everyone should pull their mask because it's just a good idea for redundancy.However, some airplanes DO carry oxygen. Aircraft that fly for extended periods over mountains extending above 10,000ft MSL and there's a point over said mountains at which the aircraft cannot reach 10,000 feet within 15 minutes (or whatever similar limit there may be for a given O2 generator), aircraft may carry oxygen in tanks, which gives more time to get to 10,000 feet. Aircraft that fly over extensive mountain ranges such as the Himalayas are the most likely to have this capability.
General aviation aircraft that are oxygen equipped typically carry tanks, because they're not used for emergencies, but they're used for preventing hypoxia in unpressurized aircraft for extended flight above 12,500 feet.
Carrying oxygen in tanks isn't effective for airlines because they usually don't need to spend a lot of time without pressure. All they need is enough to get down to 10,000 feet, and in most cases, chemical O2 generators provide this capability.
All airliners do carry SOME bottled oxygen, however, it's not for (99% of) passengers. The oxygen available in the cockpit is carried in tanks. There's also bottled oxygen in the aircraft's medical kit. Fighter aircraft have onboard O2 generators, but they're not chemical in nature.
To conclude, the overwhelming majority of commercial aircraft do NOT carry passenger bottled oxygen. While there is some bottled O2 on board, it's not available for passenger use in the case of a depressurization. That's handled by chemical generators.
