Oxygen is the final "link" in the electron transport chain because it has the highest electronegativity and affinity for electrons. This final link is key because it allows the NADH and FADH₂'s to dump their electrons and return to being NAD+ so they can pick up electrons in glycolysis and the citric acid cycle. Without oxygen, the system becomes backed up and the electron transport chain can not produce ATP (which accounts for 34 of 38 of them from a single glucose molecule).
