Life on this planet arose in an oxygen free atmosphere, even though there was plenty of water, as provided by comets etc., Anaerobic bacteria formed the first life & reigned supreme for a few billion years, it was only when Cyanobacteria evolved, that the atmosphere started becoming rich in oxygen, as they produced oxygen as a by-product, i.e. photo-synthesis.
I don't believe that planets start off with an Oxygen atmosphere, as that would mean that the planet has an oxygen rich environment at its birth, which is highly unlikely, as the prevailing temperatures, of an early life cycle planet, would bind any free oxygen radicals to other elements, locking it away.