What degrades an oil

Oxygen

Oxygen reacts with the fats and the products of that reaction are what rancidity tastes like. It happens in the headspace of a sealed bottle as well as in an open one, just more slowly, which is why fill level and how often a bottle is opened both matter. A half-empty bottle of oil is aging faster than a full one of the same age.

Light

Light accelerates the reaction, and the effect is large enough that it is the main argument for a dark bottle. This is the one of the four that the container solves outright rather than merely slows, which is why 19 of the 40 bottles that qualify for this shelf block light and why choosing one of the other 21 is a decision rather than an oversight.

Heat

Warmth speeds everything up, including this. An oil stored next to a stove or in a delivery vehicle in summer is aging at a rate that has nothing to do with the date printed on it. For a retail product that is partly out of your hands, which is an argument for a conservative date rather than an optimistic one.

Time, which is the one you cannot fix

The clock starts when the oil is pressed, not when you buy it or bottle it. A bulk oil bought cheaply because it was a year old is a product with most of its life already spent, and nothing about your bottling improves it. Ask for the harvest or pressing date and record it against the lot.

A second kind of rancidity

Free fatty acids also develop through contact with water and heat, which is a different reaction from oxidation and is what the grade standards measure. It is largely set before the oil reaches you, which makes it a reason to care about who you buy from rather than a thing to manage.

References