Oxidation in tea is a reaction of enzymes in the tea leaves with oxygen. Earlier it was also called fermentation but this term is incorrect because the fermentation process involves microorganisms. In production of pu-erh tea is fermentation used to make the tea ripe.
The oxidation is an important part in the production of tea. For green tea, the oxidation must be stopped immediately while black tea is oxidized to nearly 100%. Oolong however, has an average degree of oxidation of 10-80%.