Oddly enough, I am unable to have the tank refilled at a medical
supply shop because I don't have a prescription for O2.

One can't even get a regular pulse oxymeter without a prescription.
They make special ones for pilot that don't require a prescription.
They are identical.

Jose
--
"Never trust anything that can think for itself, if you can't see where
it keeps its brain." (chapter 10 of book 3 - Harry Potter).
for Email, make the obvious change in the address.