Hi K, be sure it is agreed in writing that the tank will be removed by the seller by a certified removal firm that wil handle the tank removal properly and also test and certify that the soil around it are a-ok. Talk with your attorney on this point - it is important - so that the agreement is clear and you are protected in case there is an unexpected issue with the removal/tsoil testing.
My recommendation to buyers considering a home with an underground oil tank is that we definitely want to have the tank removal, soil testing, and installation of a new above ground tank (if oil remains your source of fuel) taken care of by the seller. Keep this monkey off your back.
That, like everything else, can be negotiated. Hopefully, you have the seller's statement agreeing to remove the tank in writing. Then there should be no problem. You simply won't close until it has been removed.