Oxygen saturation or dissolved oxygen (DO) is a relative measure of the amount of oxygen that is dissolved or carried in a given medium. It can be measured with a dissolved oxygen probe such as an oxygen sensor or an optode in liquid media, usually water. The standard unit is milligrams per litre (mg/l) or parts per million (ppm).
Oxygen saturation can be measured regionally and non-invasively. Arterial oxygenation is commonly measured using pulse oximetry. Tissue saturation at peripheral scale can be measured using NIRS. This technique can be applied on both muscle and brain.
Oxygen in medicine 
In medicine, oxygen saturation refers to oxygenation, or when oxygen molecules (O2) 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, or (O2) stats measure the percentage of hemoglobin binding sites in the bloodstream occupied by oxygen. Fish, invertebrates, plants, and aerobic bacteria all require oxygen for respiration. Blood is also vital to the body system.
Environmental oxygen saturation 
Oxygen saturation in the environment generally refers to the amount of oxygen dissolved in the soil or bodies of water. Environmental oxygenation can be important to the sustainability of a particular ecosystem. A well-mixed body of water will be fully saturated, with approximately 10mg/L at 15 °C (here is a table of dissolved oxygen versus temperature). The optimal levels in an estuary for Dissolved Oxygen (DO) is higher than 6 ppm. Insufficient oxygen (environmental hypoxia), often caused by the decomposition of organic matter, 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.