inside the boiler, you rarely would have CO2 or O2 assuming there was pressure, it would be nearly 100% H20 vapor, did you mean the flue stack gas instead?
At the exit of the boiler, the O2% in the flue gas is usually around 5%, it could be less on a finely tuned boiler. The CO2 on a coal fired boiler is around 12%, about 8% for an oil fired boiler and about 4% for a gas fired one.
What did actually you want to know? The CO2 and O2 are removed by deaerator before the feed water enters the boiler. So the percentage is almost zero.
