Refrigeration usually slows the deterioration of our foods by slowing the chemical reactions that cause it. But extra-virgin olive oil turns out to go rancid at about the same rate in the cold as it does at room temperature. Chemists at the University of Udine in Italy found that the benefits of slowed reactions at low temperature are counterbalanced by the gradual crystallization of the more saturated oil molecules. This process leaves the remaining liquid oil with a higher proportion of vulnerable unsaturated fats, and with a smaller proportion of antioxidant substances, which tend to get trapped in the crystals. So oxidation continues at about the same pace despite the low temperature.
There's no need, then, to crowd the fridge with bottles of special olive oil. Just keep them in a cool, dark place. And this is especially true for unopened bottles. Another study from the Universities of Milan and Castilla-La Mancha reports that freshly bottled Italian and Spanish oils high in antioxidants retained much of their antioxidant capacity after as much as 240 days storage at 40 degrees Centigrade, or 104 degrees F. One oil made from the Picual olive even remained fully within the specifications for extra-virgin olive oil.
One other useful fact to be gleaned from the first report: the viscosity of olive oil nearly triples as it cools from room to refrigerator temperature. As many cooks know, you can make a plain vinaigrette very thick and creamy simply by serving it and the salad ice-cold.
Calligaris, S. et al. Influence of crystallization on the oxidative stability of extra virgin olive oil. J. Agric. Food Chem. 2006, 54, 529-35.
Lavelli, V. et al. Effect of storage on secoiridoid and tocopherol contents and antioxidant activity of monovarietal extra virgin olive oils. J. Agric. Food Chem. 2006, 54, 3002-07.