Air (just plain, ordinary air) is about 21% Oxygen, 78% Nitrogen, and 1% "other gasses." That's pumped into SCUBA tanks as is, for "normal" diving. Most of the better dive shops can add other gasses as needed for special purposes.
Oxygen has been extracted by the cryogenic process for about the last sixty years. Since it is dirt cheap to do it this way, and the process itself guarantees purity far beyond any requirements, it caught on, quickly replacing all previous methods. No other process has been used industrially for that period of time, worldwide.
Previously there were several ways to do it, but all are very expensive, and produce varying quality.
Speaking of quality, there are three levels (in the USA), leftovers from the days before the current process.
Welding Oxygen: By far the most demanding requirements, due to the often used special gasses used in welding, and the extreme purity needed to avoid flaws in the welding. ALL oxygen meets and greatly exceeds these requirements.
Medical Oxygen: The MINIMUM REQUIREMENTS are far lower. Moisture is added at the distribution point (often the bedside) with "Bubblers."
Aviator's Breathing Oxygen: The lowest requirements of all.
The term "Aviator's Breathing Oxygen" has been archaic for more than half a century, and as far as I know, it has now been eradicated from ALL Federal Regulations in the USA. It is now just "Oxygen."
Yes, there are distribution companies who will certify "Oxygen" as ABO, for a substantial additional cost. The oxygen is the same. There are NO additional tests.
I kept a large tank of "welding" oxygen in my hangar, and refilling my airplane tank was quick and easy, and the cost was pennies per fill.
Now, I'm sure CASA would toss you in jail for this, but that is a CASA thing, not based on science. My sympathies.
Best...
John Deakin