Brazing, how difficult is it?

[bulgie]

Silver solder. I would use 56% Harris cause it's the lowest melting point.

Yeah I use a lot of 56%, and with proper joint design it makes joints that are stronger than the steel.
It is very "runny" though. That makes it excellent for going inside close-fitting joints that use capillary action to suck the molten filler throughout the joint.

But if you have a joint with a small amount of surface area in contact, like two steel rods that cross, touching only at a mathematical point, I would use 40% silver, which allows you to build up a fillet around the joint. Fillet brazing is technically possible with 56%, but much more difficult. If your heat control isn't perfect, the entire fillet will sometimes slump off to one side, or even form a drip that falls to the floor. 40% is much easier for fillet-brazing and the difference in liquidus temperature isn't huge, the same equipment is used.

I actually prefer Fillet Pro from Cycle Design, which as the name implies is made just for this, but generic 40% like from Harris is close enough.
Just make sure to get the cadmium-free variant ("Safety Silv"), unless your ventilation is extremely good and you wear a proper respirator.

 

[rwm]

The O2 generators do not really produce enough pressure to get much into a bottle. You would need a secondary pump with reasonable pressure to fill the bottle and people have done this. A word of caution is in order here. You must not contaminate your high pressure pump, lines or bottle with oil. Oil with high pressure O2 would be a bomb. You would need a special pump to do this safely.