Aerobic organisms use a process called aerobic respiration to create ATP from ADP and a phosphate. Glucose (a monosaccharide) is oxidized to power the electron transport chain:
This equation is a summary of what happens in three series of biochemical reactions: glycolysis, the Krebs cycle (also known as the Citric acid cycle), and oxidative phosphorylation.
:CHO + 6 O + 38 ADP + 38 phosphate →  6 CO + 44 HO + 38 ATP
In Oxidative phosphorylation, ATP is synthesized from ADP and a phosphate using ATP synthase. ATP synthase is powered by a proton-motive force created by using the energy generated from the electron transport chain. A hydrogen ion (H) has a positive charge and if separated by a cellular membrane, it creates a difference in charge between the inside and outside of the membrane. Oxidative phosphorylation occurs in the mitochondria of eukaryotes. 
Aerobic respiration needs O because it acts as the terminal electron acceptor in prokaryotes' electron transport chain. Molecular Oxygen is reduced to water in this process.