Matt- Yer best bet is to take it to or at least call your local gas supplier. They will be able to tell you if it is an owner bottle or not. And whether or not it can be changed to a different type of gas. All the pertinent info will be stamped around the neck of the bottle. Eric
The bottles are rated for different pressure levels, and the valve attachment threads are different for inert gases (like CO2), Oxygen, or Fuel Gases. It's much more trouble to change that O2 tank over to CO2 than to exchange it for the one you need. And going the other way is quite impractical, as Oxygen tanks are extremely sensitive to any contamination at all - a little trace of oil or grease in the presence of 4,000 PSI pure Oxygen and things start going BOOM!
Take down all the info on the labels and stamped into the neck of the bottle (including the hydrotest dates and the DOT registry number) and call the welding supplies and medical supplies in your area - if there's a company name forged into the neck ring or the valve protector cap, see if you can locate them.
You might be able to get the deposit back for the tank that the other party abandoned, or if it was a customer owned tank get a credit for the empty Oxygen tank that will get you two empty (or one full) smaller CO2 tanks that you need. Worst case, a small finder's fee for helping them recover a stolen tank.
It works the other way, too - I was given two small MC Acetylene tanks, and exchanged them at my welding supply for the tank value of one B Acetylene, only had to pay for the refill contents of the B.
If you want a good source of small CO2 bottles for cheap, call your local fire extinguisher shops. If the horn or valve handles are bad (or the instructions band is damaged or lost) they cannot be used as extinguishers anymore and only have scrap value to them. But spend $25 for a CGA valve and a hydrotest, and they make fine CO2 bottles for carbonation or shielding gases.
Major no-no and I doubt if anyone will fill it for you.
The problem is "Once an oxygen cylinder, always an oxygen cylinder". Because oxygen is such a hazard if it gets put into a contaminated tank, there's a blanket imposition that nothing out of the ordinary ever happens to oxygen cylinders - not because it's dangerous itself, but because it might be dangerous if it was ever used for oxygen again.
Hmmm rules must differ according to location? Here in Southern New Mexico I found a large O2 bottle, half buried in the sand at a flea market type business. I asked how much? "$5.00". I bought it. The welding supply charged me $15 for a CO2 valve and $15 for an inspection. They even threw in the CO2 for free. I'm still MIG welding with it.
Now I agree, there is no way to go back to O2 in that bottle. There is no confusion though. The valve is for CO2 and there is a prominent sticker saying CO2.
A bit later I was given a nitrogen bottle. Again, no problem to change it over to CO2. $15 for the valve, $15 for the inspection. I have enough CO2 to weld for years.
Even later I bought a small CO2 bottle in a garage sale. I asked about changing it over to nitrogen. That wasn't going to happen! It's a one way street.