The amount of oxygen required to oxidise organic substance present in water is called

The amount of oxygen required to oxidize organic substances present in polluted water is called chemical oxygen demand (COD). 

It is an index to measure the amount of organic substances present in water. It is expressed in milligrams per litre (mg/L), also referred to as ppm (parts per million).

Dissolved oxygen (DO) is the amount of oxygen that is present in the water. It is normally measured in milligrams per liter, can also be understood as number of milligrams of oxygen dissolved in a liter of water.

The amount of oxygen used by microorganisms present in water for five days at ${20}^{°}C$ is called BOD.