Does oxygen help the environment

Higher oxygen levels in the atmosphere will lead to an increased voracity and size in those organisms that utilise oxygen as an energy source. IE. most living creatures on earth.

It is essentially volatile and toxic, however, and so increasing it beyond current levels would be unlikely to be positive.

If current levels of atmospheric Co2 however, were increased 5 fold, then plant life would thrive and be able to live in currently desolate areas such as desert and tundra ensuring an extremly abundant food supply and revitalising the planets ecosystems.