The high temperatures used to fry foods accelerates oxidation. Repeatedly using the same oil to fry foods at high temperatures (such as in factories and restaurants) leads to greater oxidation. Long shelf life food often has oil or was fried in oil that was heated to high temperatures. However, just leaving oil long enough exposed to air will also lead to oxidation, just at a slower rate (though I think highly refined oils, like those used not for cooking, have a slower or negligible oxidation rate "in air" but are usually not edible).