Oxygen-18 (, Ω) is a natural, stable isotope of oxygen and one of the environmental isotopes.
 is an important precursor for the production of fluorodeoxyglucose (FDG) used in positron emission tomography (PET). Generally, in the radiopharmaceutical industry, enriched water () is bombarded with hydrogen ions in either a cyclotron or linear accelerator, producing fluorine-18. This is then synthesized into FDG and injected into a patient. It can also be used to make an extremely heavy version of water when combined with tritium (hydrogen-3):  or . This compound has a density almost 30% greater than that of natural water.
The accurate measurements of  rely on proper procedures of analysis, sample preparation and storage.