Block I Apollo Guidance Computer 49
It may be both.
The most economical (in terms of energy, and thus $$$) way of making large amounts of O2 is to chill air, and destill it in the process. Such oxygen is liquid from the start, One advantage of having it chilled is that it is at surrounding pressure, and has its reactive force somewhat dampened.
Compressed oxygen begins to break down the oxide barreris in metals somewhere around 200+ bar. The guideline for Aluminium is to stay below 140 bar of partial pressure of O2; and for steel 250 bar. (New cylinders).
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Pushing these limits is not very worthwhile; it explodes the cost of storage. You will therefore notice that oxygen tanks have lower pressure than air-nitrogen cylinders (where 300 bar is pretty common, and 232 bar-3350 psi becoming the norm). They are therefore also somewhat lighter and bulkier. 180 bar is considered "high pressure".
Oxygen is also somewhat non-linear in storage; a liter of 180 bar O2 contains somewhat less than the 180 liters the ideal gas laws lead us to expect. (Van der Waal equations must be used). And because it is so reactive with oil it is difficult to run it through compressors etc.
Block I Apollo Guidance Computer 50
Unpure oxygen would make all kinds of havoc. In welding it is used together with argon percisely to have a pure athmosphere. Small...
Liquid oxygen is above 600 times as dense as athmospheric O2. So, there is a 7:2 advantage in density.
You still need some "diluent" gas on a normal petrol-diesel engine. Diesel engines are most sensitive. If you revv it up and feed it pure O2 the engine will break. You still need a diluent gas; and nitrous oxide fits the bill quite nicely; although it puts enourmous strain on the engine.
A 3 liter tank of NOx will only last around a minute though. It just seems longer to the driver and spectators, as they fly about. But then again the trip to space only takes 12 minutes.