Yes bound oxygen isn't rare, which is why I specified molecular oxygen. An alien race that could detect our atmosphere's composition (not outlandish, we've dete

Yes bound oxygen isn't rare, which is why I specified molecular oxygen. An alien race that could detect our atmosphere's composition (not outlandish, we've detected exoplanet atmospheres) would see the molecular oxygen and find it very intriguing (assuming they were susceptible to intrigue). If we detected an exoplanet with a 20% molecular oxygen atmosphere, it would be revolutionary. The bit about us reverting the atmosphere to the CO2 composition a couple billion years ago was a bit tongue in cheek, but not a very good job at humor on my part.