What dsaul said. I've used an oxygen concentrator for brazing for years and have written descriptions of how to set them up and use them on framebuilding forums (like Velocipede Salon). Basically I'm encouraging their use for convenience and safety. There isn't much to using them actually. Unlike dsaul I don't keep my output wide open but they are easy enough to adjust the flow. Probably his issues are related to needing to change his filter.
There are many examples of my framebuilding class students finding them cheap but there is a lot of luck to that. A sensible approach is to buy a refurbished one at
M&M Medical Equipment Repair (814) 487-7216, Beaverdale, PA 15921. They charge either $350 or sometimes $300 + shipping. I recommend the Deviliss 525. They have a greater oxygen output then other 5lpm brands. And their outlet is a B size welding hose fitting so you can screw your hose directly to the concentrator.
I'm not in my shop where I can take a picture but here are some photos of my set up. One of my students made me this great cart I can keep my entire unit on. The picture was documenting repainting some Schwinn Super Sports but you can see my Devilbiss 515 on the cart and an Invacare Perfecto 2 just happen to be sitting on the left side of the picture. In the other picture is a 525.
A Devilbiss model 525
2 oxygen concentrators just happen to be in this picture of Schwinn Super Sports being repainted.