I love all these biologists who think SP has had a poor education.
Perhaps they'd like to tell us about repiration in plants at night and where all the free oxygen got onto the planet in the first place.
Anyway you can't assume that humans have the biggest draw on the planet's oxygen reserves.
For example there is a concept of Biomass in ecology that counts the mass of a species
http://en.wikipedia.org/wiki/Biomass_%28ecolog y%29
If you added up all the krill you'd get twice the mass that humans make up.
Additionally the real ecology is underground
http://whyfiles.org/shorties/count_bact.html
you'll find 3,550,000,000,000,000,000,000,000,000,000 living organisms there.
All that biology at school with tree's and tomato plants is nice enough but doesn't amount to a hill of beans compared to the life that you can't see swarming all around us.
It's enough to give a Dettol salesman the hee-bee-jee-bees!