I don't recall the exact number off the top of my head, but yes, the critical point for O2 is safely in the realm of cryogenic.
jackssmirkingrevenge wrote:If you put 1 litre of liquid oxygen at -200°C in a container really quickly and closed the lid, it would never evaporate because there simply is too much pressure preventing most of the liquid from boiling.
Not so.
What you end up with is a supercritical fluid. The pressure is extremely high, and the density is unchanged, but it behaves like a gas, not a liquid. For a rough guesstimate, of the pressure, just use ye ol' ideal gas law, set density equal to that of LOX, and the temperature to something that approximates room temperature.
For the classic thought experiment on this (I'm probably going to screw this up, but here goes....):
Start with a pressure vessel half full of a liquid and the other half full of the same substance but in a gas form. Let's say water and steam. As you increase the temperature more water turns to steam (ie, the water boils). The steam's density increases as there is more steam occupying essentially the same volume. At the same time, the density of the water decreases slightly (hot things expand, remember?).
Heat it some more and the steam continues to get more dense while the water continues to get less dense.
Keep increasing the temperature until the density of the steam equals the density of the water.
Congratulations, you're now at the critical point.
As long as the volume of the pressure vessel remains unchanged the density of either phase can no longer change. So what does it mean to increase the temperature further? It means that your liquid phase completes it's transition to gas phase (ie, it still boils in a sense) and pressure starts increasing rapidly... But it also means that while your density is very nearly that of the room temperature liquid, you no longer have a liquid. You have a gas...but one that is referred to as a supercritical fluid for clarity.
For water, this happens at roughly 700 F, and [uh...I forgot the pressure].