If oxygen is present and the organism has the enzymes for metabolizing glucose aerobically, the pyruvate molecules get completely oxidized to carbon dioxide.  Most of the hydrogen gets removed from the pyruvates in the Citric Acid Cycle.  These hydrogens are used to reduce the hydrogen carriers NAD⁺and FAD.  Do these reduced carriers have more or less energy that the oxidized forms?  I hope you know that the reduced forms have MORE energy.  This energy is then used to build ATP in the last phase of aerobic respiration, chemiosmotic phosphorylation.  About 34 molecules of ATP are generated in this phase.  Oxygen serves as the final hydrogen and electron acceptor.   Thus, if oxygen is NOT present, chemiosmotic phosphorylation can't occur.   Check out the equations for these two processes on page 4-11 in the Study Guide.