Q: Should vegetable oil be refrigerated once the bottle has been opened?
A: It usually isn’t necessary. Natural antioxidants in vegetable oils help fight spoilage, but all oils will eventually turn rancid, developing an off smell and taste, especially when exposed to air. While some oils have a shelf life of one or more years under normal conditions, natural or unrefined oils last only about four to six months. Refrigerated oils last longer.
To keep oil fresh longer, store it away from heat, light (keep it in a dark place or in an opaque container), and air (transfer oil to smaller containers as you use it), and seal it tightly. Buy only what you will use within a few months; if you buy a larger size, you might want to refrigerate it. Flaxseed, walnut, and sesame oils have a short shelf life, so you’ll probably want to refrigerate them.
If your oil smells or tastes rancid, toss it. It may not make you sick, but if consumed regularly, oxidized fats could have undesirable cardiovascular effects. It won’t taste good anyway.
Also see Fats and Oils: Why We Love Them.