I'm in agreement, the CGA320 standard is a North American compressed gas standard for C02 tanks. Any 'standard' C02 regulator will fit the CGA320 specs. Every type of gas cylinder has a spec, I don't know what Oxygen is, but it would be something like CGA480, argon might be CGA520 and so-on. The standard includes whether it's forward/reverse thread, threads/inch, male female, material and I'm sure a few other things.
The standards are intended to protect users from using inappropriate cylinder/regulator combinations thereby endangering themselves. That said, I have heard of people adapting O2 regulators onto C02 bottles. Wouldn't advise it though.