[Donaldson Tan]

This is a spin-off from the discussion thread on Nitrogen. MO Theory predicts that oxygen is a di-radical and this agrees with observation that liquid oxygen is paramagnetic. Oxygen, despite being a radical, still exists abundantly in the earth atmosphere in its elemental form. Why?

[Borek]

There is an equilibrium between oxygen being produced by plants and being used for CO2 production. Kill all plants, wait some time (no idea how long, but oxygen cycle is probably known well enough), problem of abundant oxygen dissappears.

The other answer to your question (highly related) is that while oxygen is highly reactive kinetics of the oxidation in low temperatures (0-20 deg C) is low enough to allow oxygen level build-up in the atmosfere.

[constant thinker]

One thing about CO₂ is that rain helps to remove some of it from the air. It also removes some other things. That is a thought to keep in mind.