When olive oil is exposed to oxygen, light, and heat, it is subject to oxidation and may become rancid. Proper storage can prevent this. Depending on the quality of the oil and on how it was made, extra virgin olive oil in a sealed bottle may last from 3 months to 2 years. Click here for a more detailed discussion of storage conditions and rancidity.
As soon as you open the bottle, the oxidation process accelerates and the oil will degrade fairly rapidly. Make sure you keep your oil in a closed bottle, in a dark container or closet, away from sources of heat like your stove. You do not have to refrigerate it, although refrigeration will not hurt the oil. Buying your oil in bulk is a great way to save money. If you do so, the best way to preserve it is to keep it in a stainless steel container, called a fusti. Fusti are an excellent way to preserve olive oil. For even better preservation, you can top your fusti off with inert gas from a small spray can. If you are using a regular bulk container, try not to open it too often. Fill a couple of bottles for day-to-day use and leave the bulk container carefully closed the rest of the time.