I'm not a chemist but I'll go out on a limb and suggest a way to resolve some issues in comments.
It looks to me as though as long as you are above both the critical pressure and critical temperature at the same time, it's a supercritical fluid; thus the name.
So as long as the pressure is above 50.4 bar and the temperature is above 154.5 K (-118.6 C) it's ...
If you want to store as much oxygen in a given volume as possible, you have to increase the density significantly. There's two ways to accomplish that.
Low temperature (cyrogenic liquid)
High pressure (supercritical fluid)
If you don't need to store it for very long (say during a launch), then cyrogenic liquid oxygen has lots of benefits. You get maximum ...
The same system was used on Shuttle - allow me to discuss that, the design philosophy applies to Apollo as well (Shuttle deleted the fans though, and had a special Avoid-Apollo-13-circuit in the O2 tanks).
A supercritical fluid is any substance at a temperature and pressure
above its critical point, where distinct liquid and gas phases do not
There was no "slush" in the Apollo cryo tanks. The O2 and H2 in the tanks were stored at conditions that made them supercritical fluids. The critical pressure for O2 is ~ 730 psi, the Apollo tanks were at ~ 900 psi. (H2 critical pressure is ~187 psi).
from Apollo Operations Handbook Block II Spacecraft , emphasis mine
These supercritical fluids tended to ...
From this NASA pdf:
At 46:40:02 Mission Elapsed Time during the Apollo 13 mission, both
oxygen tank fans were powered on, hopefully to get a more accurate
reading of the tank pressure.
In fact, the pressure is not influenced by fan operation. The reading of the capacitance tank gauge depends on a uniform temperature of tank contents.
56 hours into ...