Ed Pawlowski wrote:

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.

Even the large tanks (5-foot tall) are used in 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.

So what you're saying is that welding tanks are probably safer - because
they're not as exposed to such nasty conditions and situations as the
home-use medical tanks are? And welding tanks are more likely to be
returned with some positive pressure - as opposed to home-use medical
tanks as you have just described?

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.

So go further and tell us if welding tanks are, or are not, evacuated
prior to being refilled.