If the tank was full of acelelene at atmospheric pressure before filling with oxygen the ppm od acelelene in the breathing gasses would be the absolute least of your problems, as pure acetelene in it's free form is extremely unstable at anything over about 16psi. Combine that characteristic of acetelene with 100% oxygen environment and you won't have a TANK, much-less an unbreathable mixture.
Also, putting CO into a highly pressurized pure oxygen atmosphere for any time would LIKELY end up with an exothermic reaction producing CO2.