"Some Guy" wrote
That is exactly why medical oxygen thanks have to be evacuated.
Rare that one would come back with any pressure at all.

I would think the contrary.

O2 tanks used by hospitals are more likely to be part of a manifold
system, and as such will always be maintained at some positive pressure
by virtue of the fact that at least one of their "gang-mates" is likely
to have enough excess pressure to keep them partially pressurized.


You think wrong. Medical oxygen is used in many places aside from
hospitals. Thousands of bottle every day are used in private homes. They
are single size, no manifolds, They are generally used until empty.
Valves are left open, regulators removed. They are sometimes stored in poor
environments, must basements, trunk of a car, under the sink, laundry room.




If the strongest argument so far is that a "medical-grade" tank of O2
has lived it's life with minimal to zero infiltration of atmospheric
humidity (or even nitrogen) compared to a welding tank, then that's a
pretty weak argument to say that a tank of welding O2 is unhealthy to
breath. Last I checked, we all take in some some water vapor and
nitrogen when we breath standard air.

In other words, a lack of "purity" does not equal unhealthy or hazardous
for human breathing. A lack of purity (it seems) will degrade welding
performance, maybe mess up equipment, etc.




And so far, it's just been pure speculation here that tanks of welding
O2 are *not* evacuated prior to filling, just as supposedly medical
tanks are.

Not speculation. I've filled tanks. I followed the regulations.