Little amounts of oil can be disposed of easily, but you may wonder what to do with the bottles of oil that have gone rancid or the larger amounts of cooking oil that were used for deep frying. Don't put the oil down the drain -- any drain -- as it will stick to pipes and cause blockages. There are much better ways to put that oil to use that help your garden and the environment.
Use old oil in your garden. For an organic insect pest spray, mix one cup of your unwanted oil with three tbsp. of Murphy's oil soap or baby shampoo and add 1 c. warm water. If you add garlic or hot pepper, it will be an even better deterrent to future insect pests. Wash your sprayer clean with soap and hot water when done.
Pour a cup at a time into your compost heap. Since it is a vegetable product it will decompose safely. But because of its viscosity, vegetable oil is slow to break down so you don't want to smother any parts of your compost pile by using too much at any one time.
Donate the oil to centers that accept vegetable oils to convert to bio-diesel fuel. Some cities have programs in place to help collect oil for recycling, other centers operate privately.