Are there any proposed projects that could detect such life signatures as, say, an oxygen atmosphere in an extrasolar planet?

I think an oxygen rich atmosphere would be a slam dunk proof of life on an EP. A carbon dioxide rich atmosphere would also be very interesting, but not quite compelling - e.g., a planet undergoing intense volcanic activity could possess a CO2 enriched atmosphere in the absence of life. An oxygen rich atmosphere [~20%] is, however, hugely improbable. Oxygen is highly reactive and would be rapidly depleted without a renewal mechanism. Plant life is the only viable mechanism I can think of that would explain this.

Water worlds could develop oxygen atmospheres as hydrogen is split off water molecules and escapes into space. If the water is deep enough there might be a deep enough layer of "warm" ice created by pressure to prevent free oxygen being absorbed by minerals or minerals could absorb enough oxygen to reach an equilbrium. But aparently you could tell from the spectrograph if it's likely to be a water world or not. How to tell if a water world has life? The presence of methane should show that. (However, it has been suggested that traces of methane in the atmophere of mars could be natural in origin.)

And I have no idea of when we'll be able to get a spectrograph of an exoplanet.

First, I think, we're trying to identify the earth-like planets, which the SIM Planet Quest and Terrestrial Planet Finder missions are intended to do; planned to launch sometime during the next decade.

I suspect we'll discover something before then though...

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