by International Olive Oil Council law, EU law, and the California Olive Oil Council seal, nothing can be added to olives to make olive oil. Olive oil is high in natural antioxidants.
The Extra Virgin definition:
"This oil is obtained only from the olive, the fruit of the olive tree, using solely mechanical or other physical means in conditions, particularly thermal conditions, which do not alter the oil in any way. It has not undergone any treatment other than washing, decanting, centrifuging and filtering. It excludes oils obtained by the use of solvents or re-esterification methods, and those mixed with oils from other sources."
Nearly 100% of seed oils such as corn, canola, sunflower and safflower are extracted with chemicals. Many oils, especially nut oils have had stabilizers and antioxidants added to them to extend their normally short shelf life. During storage in large tanks, inert gas is often added to the top of vegetable oils to prevent oxidation. These gasses do not end up in the bottle however and do not combine with the oil.