Vegetable oil can -- and does -- go bad after the expiration date or even before that date if it's not stored properly. Think of the date on the bottle as a suggestion, rather than a hard and fast rule. You can safely use the oil as long as it smells and tastes fresh.
What the Labeling Means
Most vegetable oils don't contain an expiration date, but rather, a "best if used by" date. This label simply means that the oil may still be safe to use after the date on the bottle, but it will be at its best quality before that date. You can use the vegetable oil up to one year past the date on the bottle if it has been stored properly.
Determining Freshness Yourself
The date on the bottle offers an approximate guideline for how long the oil will stay fresh, but your own observations and record keeping are more accurate methods to gauge the oil's freshness. In general, vegetable oil remains fresh for six months after opening or up to one year unopened. Vegetable oil may change color or become slightly cloudy as it sits, but if you notice a sharp, bitter flavor or odor, the oil is probably rancid. Rely on your sense of smell and taste to determine when an oil has gone bad.
The three enemies in food storage are light, air and heat. To keep vegetable oil as fresh as possible, store it in a cool, dark cupboard or pantry away from heat. Don't store it near the oven or stovetop. Pour out only what you need at one time and keep the oil tightly capped. Some oils, such as olive oil, sesame oil, peanut oil and walnut oil, are especially prone to rancidity. Store them in the refrigerator.
Type of Oil
How long your oil stays fresh depends partly on the type of oil. The term "vegetable oil" is a bit confusing, since most oils are made from some type of vegetable. A product labeled vegetable oil may contain blends of grape seed oil, corn oil and canola oil, but most vegetable oil is soybean oil. Blended oils typically stay fresh longer than single oils -- up to two years past the date on the label. Sunflower and safflower oils stay fresh for up to two years, as well, while corn oil and vegetable oil should be used within one year.