Can you use copper pipe for oxygen?

Yes, you absolutely may use copper pipe for Oxygen service. There are, however, several things that must be taken in to consideration.

First of all, any piping that will carry Oxygen MUST be cleaned in accordance with CGA G-4.1. If there is any oil / hydrocarbon residue in the pipe and pressurized oxygen hits it with any force there is a high probability that the oil may ignite. You should purchase tubing and fittings that are cleaned and bagged for Oxygen service.

Second, you need to keep in mind that copper tubing is for relatively low pressures. The pressure rating is also dependent upon the method used to join the copper.

If the copper is intended for cryogenic oxygen service, a high silver content brazing alloy should be used. Also, whenever brazing you should have a constant purge of dry nitrogen flowing through the pipe - this will prevent oxidation / scale inside the pipe.

After assembly you should pressurize the system with Nitrogen and use a bubbling leak detector that is OXYGEN SAFE to check for leaks. Whatever method is used to join the pipe, you must be certain that there is no contamination (oil, flux, etc) inside the pipe before the system is pressurized with Oxygen.