The tank must be taken out of service and properly closed. This involves emptying the tank and pipes of any oil and cleaning it carefully in order to remove any residual oil and vapors. If there's not sign that the tank leaked, it's then considered closed and is ready for removal.
The tank is then removed. Above ground oil tanks can then be disconnected, put on a trailer, and hauled away to the proper disposal location. Underground oil tanks must be dug out of the ground using an excavator, lifted onto a trailer, then hauled away to the nearest landfill or recycling facility.