We have a pretty good idea of what life would be without oxygen because life started out without oxygen and there are still organisms that don’t need it. There are still many species of bacteria, fungi, and other single-celled organisms that don’t need oxygen, or oxygen is even toxic to them. We need oxygen to make a lot of energy. Oxygen doesn’t actually create energy, since energy is conserved, but instead makes a high-energy compounds that can be used for functions later. Organisms, can still make energy without oxygen, but they only make about one tenth as much energy per sugar molecule. The reason oxygen is so useful for making energy is that it strongly attracts electrons. Therefore, as electrons move from sugars to oxygen, they create energy, in a sense like how water going over a water wheel makes electricity. Since sugars have multiple carbons, the most “biologically broken up” they can be is into CO₂ since that has only one carbon. Therefore, having oxygen allows an organism to break up its carbon containing sugars as much as possible to extract as much energy as possible. So large organisms, complex organisms generally require more energy so they need to use oxygen. Without oxygen, all organisms would likely be microscopic and not very complex.