There are various "grades" of Oxygen.
The only one regulated in the US is "Oxygen USP", which is a medical gas and requires a prescription to obtain and use.
"Avaitor's Breathing Oxygen" is, as the name implies, intended for human consumption (typically by owners of unpressurized light aircraft.) It is also what most dive shops typically will obtain and use for mixing PP fills of Nitrox and Trimix.
It is NOT a prescription drug and is perfectly legal to obtain and use for diving purposes, either at or below the surface of the water.
(That's what I have on my boat.)
There are also laboratory grades (which instead of being 99.5% O2 might be "five nines" or "eight nines" pure), and welding gas, which is at least 99.5% pure as well - it has to be for oxy cutting torches - but the cylinders and such used to store it could conceivably have contaminants in them that would make it unacceptable for breathing purposes. The lab grades are gross overkill (and overly expensive) for diving use.
The only one regulated in the US is "Oxygen USP", which is a medical gas and requires a prescription to obtain and use.
"Avaitor's Breathing Oxygen" is, as the name implies, intended for human consumption (typically by owners of unpressurized light aircraft.) It is also what most dive shops typically will obtain and use for mixing PP fills of Nitrox and Trimix.
It is NOT a prescription drug and is perfectly legal to obtain and use for diving purposes, either at or below the surface of the water.
(That's what I have on my boat.)
There are also laboratory grades (which instead of being 99.5% O2 might be "five nines" or "eight nines" pure), and welding gas, which is at least 99.5% pure as well - it has to be for oxy cutting torches - but the cylinders and such used to store it could conceivably have contaminants in them that would make it unacceptable for breathing purposes. The lab grades are gross overkill (and overly expensive) for diving use.