DEQ instituted new rules for dealing with old oil tanks this year. If it is determined that a property has had an underground oil tank, it must be tested by a DEQ-approved contractor to ensure that it is not leaking. If it is leaking, owners can be held responsible for decommissioning and soil contamination abatement. Lending institutions don't want to get caught in the middle of this. This concern has made oil tank disclosure a major real estate concern in Oregon, even in areas where it isn't really applicable. Underground oil tanks were quite common in the Portland and Salem areas in homes built before 1970 or so, but not on the coast.
I think it highly unlikely (though not impossible) that you would have an underground oil tank on a coastal property unless there was a local vendor for heating oil in the Seaside area (again, unlikely). You need to have a sizable population base to make a heating oil business profitable. Your home has probably always been either all-electric or partly propane fueled. Whether or not the bank knows anything about the history of the home, I believe they are still obligated to provide you with a disclosure form. Obviously you can't prove a negative. In the absence of any obvious indicators of an old oil tank, I think you and the lender can rest easy. Look for:
1. A vent or fill-pipe outside of the home
2. oil supply lines in a basement wall or crawl space
Here is a link with more information...probably way more than you care to digest.
Oregon DEQ: Land Quality - Tanks - Heating Oil Tank Program