Oxygen on exoplanets

I've been reading lately about the proposed OWL telescope, which would 
have sufficient resolution to make direct observations of exoplanets. This 
would enable scientists to analyse their atmospheres spectroscopically and 
determine their composition.

Now I understand that the presence of a gas like oxygen would be a strong 
indicator of the presence of life, since oxygen is very reactive and won't 
last in the atmosphere unless there is some process constantly 
replenishing it. Life is obviously such a process, but does anyone know if 
there are others, already known or speculative? Would the presence of 
oxygen be enough to allow us to safely infer the presence of life?

Thanks,
Seán

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