Biochemical Oxygen Demand (BOD), is a measure of organic material present in water. BOD value less than 5 ppm indicates a water sample to be

1.  Rich in dissolved oxygen 

2.  Poor in dissolved oxygen

3.  Highly polluted

4.  Not suitable for aquatic life

The total amount of oxygen consumed by microorganisms (bacteria) in decomposing organic matter present in certain volume of a sample of water is called Biochemical Oxygen Demand (BOD) of the water.
Water considered to be pure if it has BOD less than 5 ppm whereas highly polluted water has BOD more than 17 ppm.
Thus, water having BOD less than 5 ppm is rich in dissolved oxygen.