Before you buy: At the grocery store, reach for a bottle at the back of the shelf, where the oil is shielded from fluorescent light.
To store at home: Keep your oil in a low, cool, dark cupboard, or in the refrigerator if you use it infrequently―never next to the stove. If you buy oil in clear glass, wrap the bottle in foil when you get home, and keep it covered.
If you buy in bulk: Lugging home a gallon-sized container? Do what they do in the Mediterranean: Pour some oil into a clean dark wine bottle fitted with a pouring spout.