In medicine, oxygen saturation refers to oxygenation, or when oxygen molecules (O
2) enter the tissues of the body. In this case blood is oxygenated in the lungs, where oxygen molecules travel from the air and into the blood. Oxygen saturation ((O
2) sats) is a measure the percentage of hemoglobin binding sites in the bloodstream occupied by oxygen. Fish, invertebrates, plants, and aerobic bacteria all require oxygen for respiration.
In environmental science
In aquatic environments, oxygen saturation is a ratio of the concentration of dissolved oxygen (O2) in the water to the maximum amount of oxygen that will dissolve in the water at that temperature and pressure under stable equilibrium. Well-aerated water (such as a fast-moving stream) without oxygen producers or consumers is 100 % saturated. 
It is possible for stagnant water to become somewhat supersaturated with oxygen (i.e. reach more than 100 % saturation) either because of the presence of photosynthetic aquatic oxygen producers or because of a slow equilibration after a change of atmospheric conditions. Stagnant water in the presence of decaying matter will typically have an oxygen concentration much less than 100 %.
Environmental oxygenation can be important to the sustainability of a particular ecosystem. Refer to ( for a table of maximum equilibrium dissolved oxygen concentration versus temperature at atmospheric pressure. The optimal levels in an estuary for dissolved oxygen is higher than 6 ppm. Insufficient oxygen (environmental hypoxia), often caused by the decomposition of organic matter and/or nutrient pollution, may occur in bodies of water such as ponds and rivers, tending to suppress the presence of aerobic organisms such as fish. Deoxygenation increases the relative population of anaerobic organisms such as plants and some bacteria, resulting in fish kills and other adverse events. The net effect is to alter the balance of nature by increasing the concentration of anaerobic over aerobic species.
- "Dissolved Oxygen - Environmental Measurement Systems". Environmental Measurement Systems (in en-US). Retrieved 2015-10-08.
- "Environmental Dissolved Oxygen Values Above 100% Air Saturation" (PDF). IOOS Website. YSI Environmental. Retrieved 29 July 2015.