C+O2=CO2
So as we see an increase in CO2 in our atmosphere shouldn't we also see a related decrease in the oxygen levels.
Oxygen is given as making up 19-21% of the Earths' atmosphere but it can vary considerably locally. At levels below 15% it becomes increasingly difficult to breath.
Should we be worried about climate change caused by increases in CO2 levles, or the resultant decrease in O2 levels which could have a more immediate effect?
http://www.trufax.org/general/oxygen.html