It’s safe if you know what you’re doing but they’ve no way of knowing whether you do or not. Years back, our office aircon supplier was changing a system over to the newer (less environmentally harmful) refrigerant, a job they’d done on many many systems. Their SOP required the system to be purged with nitrogen but, on this occasion, they connected an oxygen cylinder by mistake. Needless to say, there was an explosion, serious injuries and a lot of damage. Fortunately no fatalities.
In addition to the need for oil free fittings, a restrictor (small hole) on the supply side is a good plan, as it limits the rate of release of gas (and therefore energy release) if there is a leak.
I think the danger of grease is greatly over-stated; see the main oxygen thread.
There is no need for any lube on oxygen fittings.
What must be watched is that the fittings and hoses are in a good condition, and the threads are undamaged and properly tightened up before the cylinder valve(s) is/are opened. You don’t want some complete monkey doing this, because if a fitting comes off, it will fly off at something like mach 1. Air powered air rifles easily manage mach 1 (that’s the bottom-end 30 ft-lbs level) and some do much more, and they use 3000psi air, and they have a regulator mechanism which maintains ~ mach 1 even with perhaps 1/2 the pressure.
Another thing is that a fresh cylinder will be 3000psi (200 bar) whereas the typical aviation cylinders are redlined at 150bar. I don’t know why this is, especially as most of the cylinders are scuba ones, relabelled and sold for 5x the money. So if you just join them up with a hose, open up both valves and walk away, you probably want to walk back to it with some care 
In practice it won’t explode (the pressure test goes way higher than 200 bar) but you will end up way over the redline, and it’s probably not a good idea to be putting 200bar up the 1st stage regulator which is expecting 150bar. So you have to let some air out.
AeroPlus wrote:
So what is the story?
The story is that he’s BS-ing you. I’ve been trans-filing oxygen that way for 20 years. The only important things are that the equipment (tanks, adapters, hoses, fittings, o-rings) is oxygen compatible and everything used in the process is oxygen clean (no grease, or oxygen-compatible grease).
I thought so that he was BS-ing me. Thanks for the info.
When using medical oxygen either from the bottles or a generator you are given all sorts of warnings not to use any creams or anything else containing petroleum jelly.
Common mistake people make is using wrong o-rings (e.g. silicon instead of viton or other oxygen material) or wrong grease (like silicon based instead of chrystolube). Other than that degreasing, and washing with detergent before exposing to oxygen.
Where do you buy the appropriate fittings and tubings in Europe?
My case is to refill my 9/16 male fitting of the aircraft with one or more large welding bottles.
Thanks
Is this relevant?
Also my original writeup (search for “hose assembly”).
You can buy it e.g. here. www.flugversand.de
I can’t see anything of relevance at that URL. Just some standard O2 kits.
On the earlier posts, I had constant hassle getting O2 refills, due to anally retarded staff at scuba shops. I could be getting refills for a year and then some new dumbass in the shop would look at me and say “you are not a diver are you”. After a few years of this I got my own rented cylinder, as described in the link I posted above.
I’ve never heard of a “9/16 male fitting” – is this one of the fittings used to refill a fitted O2 system? If so it may be detailed at that link I posted above (bizjet O2 fittings).
