Oil furnaces require oil tanks to store fuel. Generally there is a lifetime for the oil tank before rust eat through the tank causes leakage problem. Rusting occuring when water droplets condense at the bottom of the tank and starts to rust.
When oil leaks, it will contaminated the surround soils and is bad for the envirnoment, a complete clean up (remove all the contaminated soils) is required by the law. This will translate to a very expense clean up operation, in some cases, the house may need to tear down to make way for the clean up.
When you buy a home with oil tank, make sure the tank is located in the place that is easily accessible for maintainance or replacement (some older houses built such a way that the oil tanks are buired under ground, and it will be very expensive to do any maintainance works)
Also, pay extra attention to the tank condition. Check if there is any sign of leakage problem. If there is a leak and you are the owner of the house, you are the first person financially responsible for the clean up operations, eventhough the leaks has been occured prior to your ownership, and it will be up to you to prove this in court and get the previous owner share the responsibility.
The next thing is to find out how much does it cost to insure the house with oil tank. Generally, the insurance company will send an adjuster to evalute the condition of the oil tank. If they found there are potential problems, they will ask the owner to fix the problems before they can insure the house.