In article ,
Matt Whiting wrote:

Don Hammer wrote:
Legally you have to use ABO (Aviator's Breathing Oxygen). The only
difference is supposed the quality assurance for contaminants such as
water. My oxygen supplier gets them from the same source and tests it
all to better than ABO standards. He ships bottles marked ABO to
those who require it. Check with some of your local suppliers and
they'll certainly be cheaper than any FBO.

Which regulation requires this?

Matt

I wouldn't be surprised if it's in some part 121/135 op-specs, which is
as good as being a regulation.