Iron exposed to the atmosphere. That is, either on the surface or washed down into the ocean from immense basaltic deposits. The oxygen produced as waste by anaerobic microorganisms would first bind with the oceanic iron, then as it became depleted the oxygen would bubble to the surface and enter the atmosphere. It would have to combine with surface iron and any other active molecules before it could accumulate in quantity.
To this day, the vast majority of Earth's free oxygen is produced by ocean microorganisms.
OK, that makes more sense.