When we bought our house 29 years ago, we were assured that the oil tank had been decommissioned. BUT there was no paperwork.

About 6-7 years later, my wife (who is a licensed environmental engineer and certified hazardous materials manager) noted that the NJ laws were changing and likely to become much stricter and onerous on homeowners. IIRC, “decommissioning” was no longer going to be possible. So we bit the bullet and had the tank removed completely (not cheap). AAAaaand… it had not been decommissioned. Not at all. Goad we had it taken care of.

My advice: if you do not have legal paperwork from a licensed contractor, the tank is not decommissioned.