It is not a reason to panic.

You can easily find information on-line regarding leaky oil tanks and its removal/clean up.  I personally have gone through this process, and it was not as scary as some of you would think.  If there is a good reason for you to buy this property, I don't think you have to give it up simply because of a leaky oil tank.  At a minimum, you could try to ask the seller to remove the tank, do the clean up, and provide you with the requried paperwork.