The answer is: it depends. Did you have a home inspection upon purchasing the home and did that inspection include the heating oil tank? When was the leak discovered?
When you purchase a house there can be defects which are detected by inspection and then there are things known as latent defects, which typically are not revealed upon a home inspection. If there are latent defects then the question becomes whether the people selling the house had reason to know of those defects before you can attribute responsibility to the sellers.
DISCLAIMER: Brandy A. Peeples is licensed to practice law in the State of Maryland. This answer is being provided for informational purposes only and the laws of your jurisdiction may differ. This answer based on general legal principles and is not intended for the purpose of providing specific legal advice or opinions. Under no circumstances does this answer constitute the establishment of an attorney-client relationship. For legal advice relating to your specific situation, I strongly urge you to consult with an attorney in your area. NO COMMUNICATIONS WITH ME ARE TO BE CONSTRUED AS ARISING FROM AN ATTORNEY-CLIENT RELATIONSHIP AND NO ATTORNEY-CLIENT RELATIONSHIP WILL BE ESTABLISHED WITH ME UNLESS I HAVE EXPRESSLY AGREED TO UNDERTAKE YOUR REPRESENTATION, WHICH INCLUDES THE EXECUTION OF A WRITTEN AGREEMENT OF RETAINER.
You need to look at your purchase contract and the disclosures the sellers made, including any standard real estate disclosure forms. In Maryland, certain known defects must be disclosed, but when a property is sold "as is" and no required/voluntary disclosures are required or made, then you may be stuck with the repair on your own. However, this sounds like an expensive repair, so I would take all your contract documents down to a real estate lawyer to review. If the sellers knew of the leak, and if they misrepresented their knowledge or failed to make a legally required disclosure, then you would have recourse. When looking at the property, did the question ever arise regarding the condition of thee oil tank, and did the seller make an representations to you? Is there any evidence that the leak was known prior to the house selling or your making an offer to purchase? You should also confirm when the leak likely occurred. Your homeowner's insurance policy may cover this. Also, many new owners obtain new home warranty policies, and many times these policies are included by the seller as part of the sales contract to entice more buyers. Did you get such a policy? Review your documents.
And another thing to consider: If the leaking tank on your property damages your neighbors (imagine that you contaminate a neighboring well, or a public stream or tributary), then you will be primarily responsible to your neighbor and even the government.
Now that you know about the leak, you are on "notice" of the potential harm to others and the environment.