[DeadParrot]

Might matter if you are talking a "Age of Dinosaurs" or "Current Times" loss of life. Age of Dinosaurs only has to deal with biomass degenerating without the aid of any critters. Current times has the same problem plus all of the millions of gallons of chemicals and oil products normally kept in storage. Add in all of the nuclear power plants that might react poorly to loss of human supervision. By the time your space explorers arrive, possible there could be plenty of O2 but with deadly levels of radiation and toxic chemicals.