Can I use oxygen to fill my nitrogen tank?
The short answer is no and this falls back onto the rule of not filling your tank with anything it's not rated for. Oxygen supports combustion and any kind of spark or flame could cause the tank to combust, this is why you see the big, red flame stickers on oxygen bottles. Also, when you fill a nitrogen tank it heats up because of all the pressure being applied to the bottle. Oxygen and pressure do not mix because there is a good chance it will cause the tank to rupture and ignite ontop of that.
Also if there is any grease or oil present on the tank reg or inside the reg, there usually is some kind of grease or oil for the valve, the heat and oxygen can cause those to ignite as well. So don't try to fill up your tank with oxygen from a welders torch or grandmas' oxygen supply because chances are not only will you blow yourself up but you'll light yourself on fire ontop of it. That's the grim reality.
The end result of a Rapid 7 pre-charged air rifle being filled with pure oxygen and then fired - not pretty. The owner ended up in intensive care, and is lucky to be alive. Uziel.