Do you store it near the stove? …or another hot spot?
If so, the heat could be draining the oil’s antioxidants and speeding up the spoilage process. Move it to a cooler spot in the kitchen.
Do you leave it on the counter?
Another mistake: Light damages cooking oil. Store it in a dark cupboard in an opaque container.
Did you buy it ages ago?
As a rule, vegetable oils stay fresh for about 6 months or so, while olive oil keeps for about a year. Now and then, give a bottle of cooking oil the sniff test. If it has an off-odor, it’s probably turned bad, so toss it out. If you buy a large container of cooking oil, pour a pint or so into a smaller bottle for everyday use. Avoid opening the original container too often.
Do you keep your oil in the refrigerator?
No problem. Some cooking oils must be kept cool to stave off spoilage, such as walnut oil and almond oil. However, storing any oil in the fridge will keep it fresh longer. The oil may turn solid, but take it out of the refrigerator shortly before cooking and it will reliquefy.
Do you pour old cooking oil down the drain?
If so, do you know a good plumber? You’ll have a clogged sink before long. Put old or used cooking oil in a leak-proof container and throw it out with the trash.