that is an interesting pressure regulator setup. it has a hard plumbed inline secondary regulator with a high(er) pressure gauge for the secondary, and then it looks like two outputs with one having a check valve.
if the regulator bodies are the same, i would say to actually make it into two separate primary regulators, just buy the tank fittings.
i believe you'll be fine using it as a co2 regulator, even if it was previously an oxygen regulator. the fitting is pretty easy to change. you shouldn't experience any contamination as oxygen regulators really need to be cleaner than other gasses due to the oxidation of materials (oil will combust with very little energy input when in a pure oxygen enviornment).
just so you know though - i would never use the regulator as an oxygen regulator again after converting it for use with co2. even though there shouldn't be anything in the co2, oxygen is EXTREMELY dangerous, and i would not recommend anyone to take a chance using it for oxygen again.
just a thought - check the max inlet pressure on the regulator body, usually there is a sticker. i forgot the pressure oxygen is stored at in tanks, but i assume it would be higher than co2, so you should be ok.
in case you didn't know - co2 fittings are CGA-320. i believe oxygen is 540? i know nitrogen is 580. just make sure to get the right fitting.
