Yesterday, I ran a half-day olive oil tour. As always, we explained all the factors that go into making a fine extra virgin olive oil. As important as making the oil is how you preserve it before it goes on sale. First of all, your need to store the oil in stainless steel tanks that are in a temperature-controlled environment of around 18-20ºC. Top producers pump nitrogen into the oil which goes to the surface displacing the air to form a blanket which will inhibit oxidisation. Olive oil is then only bottled as orders come in. To reduce oxidisation in the bottle the mill that I visit introduce a small amount of nitrogen into each bottle before they are filled (see photo below). The means there is no air in the space above the oil in the bottle. All fine olive oil should be put in dark bottles or tins so that the oil does not deteriorate with exposure to light.
When you buy fine extra virgin olive oil make sure it is from the latest harvest and always store it in a cool place at home. Also, you should always screw the top back on the bottle when you are not using it. Olive oil can easily absorb aromas which of course are ever-present in a well-used kitchen.