What is the oxidation of olive oil?

Oxidation is the reaction of oxygen in the air with unsaturated fatty acids in olive oil.

It is one of the primary causes of the deterioration of olive oil freshness and quality.

The regular oxygen molecule we breathe is a less energetic triplet oxygen and may not directly react with unsaturated fatty acids in oil. When excited by light, heat, or certain metals, the triplet oxygen converts to a singlet oxygen, which is highly energetic, unstable, and always ready to react. (Orientation of unpaired electrons in the oxygen molecule determines singlet vs triplet status)

In the presence of light, the photosensitive pigment found in olive oil, chlorophyll, stimulates the conversion of triplet oxygen to singlet oxygen at a rapid rate. The singlet oxygen thus formed fiercely attacks polyunsaturated fatty acid leading to the formation of hydroperoxides. 

This is the primary oxidation of olive oil. The hydroperoxides are very unstable and break down to small volatile molecules, such as aldehydes, ketones, alcohols, acids, hydrocarbons, and esters. These small molecules produce off-aroma in olive oil, indicative of bad oil quality/ rancidity.