What is the difference between oxidation and fermentation?

Oxidation is a natural phenomenon that causes any harvested plant to fade and oxidize. Reacting with the oxygen in the air the pigments of tea leaves begin to turn brown. 

Fermentation is natural or accelerated aging of tea leaves by the influence of microbial activity from several different types of bacteria, which develops the characteristic flavor of dark tea. Among six types of tea, only dark tea (also known as pu-erh) is fermented.