In general cooking oil can be re-used. The reason we use oil for cooking is that it boils at a much higher temperature than most of the other ingredients we use in food. So we can heat up oil to a very high temperature, put food in it (like some vegetables, for example), and the hot oil will cook the food without boiling off. Heating up the oil usually does not change it chemically, so any oil that is left over can be used a second time.

Olive oil is a little bit different from other cooking oil (like canola or peanut oil), though. Olives makes several different kinds of oils, so olive oil is usually a mixture of all of these different types. Some of the oils burn at much lower temperatures than the others, so what can happen with olive oil is that when it heats up, some of the oils in the mixture start to burn before the rest of the oil starts to boil. When they burn, the olive oil doesn't taste right anymore. Other oils, like canola oil, do not usually contain as many different kinds of oil, so this is not a big problem. Apparently, the most expensive types of olive oil (the "extra virgin" variety) starts to burn at around 350 degrees Fahrenheit. So if you get the oil that hot, it is not as good to re-use it. From personal experience, I can tell you that you know when the oil is burning because it gets very smoky. Plus it doesn't taste as good after that!

It is possible to buy a different kind of olive oil, called "refined olive oil" that does not burn as easily because some of the easy-to- burn oils have been removed. This kind of olive oil can be re-used a few times, although eventually it starts to get burned as well, so you shouldn't use it more than about 6 times.

By the way, I used wikipedia to help me answer your question, and that site has lot of other information about olive oil: olive_oil

As long the olive oil was not burning (smoking and turning brown) and all food particles (from whatever you were cooking) could be removed, I think you could reuse the olive oil.

In reality, the olive oil probably burns a little every time.Also, It is impossible to remove all of the food from the olive oil. Small pieces of food will break down or crumble off and be stuck in the olive oil. These pieces of food will continue to cook and eventually turn the oil black with partially burnt food particles. Your oil would start to taste really bad and possibly be unhealthy.

One example is the use of deep fryers at fast food restaurants for cooking french fries. The oil is reused many times as potatoes are submerged in the hot oil and then removed, and fresh potatoes are again added and removed. Eventually though, the oil gets very dirty from the food particles and the oil burning. The oil goes from a clear amber color to a darker black color and is replaced with fresh oil.

So, in short, cooking oils can be reused a little bit, but it depends on what you are cooking.