What Kurt said... Today Nitrox is much more popular than years ago. I owned/operated a dive shop back in the 1990's when it was still brand new voodo and was the first person in my neck of the woods to be using Nitrox and Trimix. We had O2 on hand for filling cylinders for decompression stops, and for enriching air to Nitrox. Additionally we had Helium on hand for making Tri-mix gasses for deep diving.
The benefit of all that prior experience is that while I sold everything and got out of that activity as a profession, I still own a 4-cylinder Oxygen fill station in my hangar. O2 fills are dirt cheap if you own a couple of the large cylinders. It's fairly trivial to purchase the required hardware, and it basically costs a similar price to fill one gigantic cylinder at the gas store than it does to have one small portable cylinder filled elsewhere. With my 4-cylinder cascade system, I haven't even needed to top off a bottle in quite some time.
I'd recommend that if you use O2, and you have others near you that also wish to do the same, that you partner up and buy yourself 3 cylinders with the proper valves, and a fill whip. It's not necessary to have cylinder interconnects. Just get the handwheel type connectors and spin it on the lowest pressure cylinder first. Fill as far as you can. Move it to the next cylinder and fill it some more. Then finish with the highest pressure cylinder. When your fills aren't sufficient any longer, take the lowest pressure one and get it filled and make it the highest one again.
Cheap and simple. Just make sure everything you use is Oxygen clean and compatible and keep it that way.
Oh, and always open valves slowly with O2. Adiabatic compression is a real thing. You can even feel extreme heat when quickly pressurizing air lines. It's pretty amazing when you feel it first hand.