Dissolved Oxygen (D.O.) is the amount of oxygen gas dissolved in a given quantity of solvent, usually water, at a given temperature and atmospheric pressure. It is usually expressed as a concentration in parts per million (ppm) or mg/L. For trace level of dissolved oxygen, parts per billion or µg/L is the unit typically used. D.O. The concentration can also be expressed as percent saturation, where saturation is the maximum amount of oxygen that can theoretically be dissolved in water at a given pressure and temperature.