In Wisconsin any fuel oil tank stored outdoors, in a basement or in any other building that is not being used (no fuel oil or other fuel consuming device actively working). must be properly decommissioned.
There are two options for decommissioning:
(a) The tank and all connected piping, including the vent and fill piping, shall be emptied, cleaned and removed from the premises.
(b) 1. The tank and all connected piping shall be emptied and purged of all vapors. 2. If the tank is not removed, the tank vent shall be left intact and open.
3. If the fill pipe is not removed, it shall be filled to the top with concrete and capped.
4. Any piping that is not removed, other than a tank vent, shall be capped or otherwise sealed.
This post references information from the following Wisconsin Legislative Code, please refer to this code an any applicable local or municipal codes for additional information. https://docs.legis.wisconsin.gov/code/admin_code/atcp/090/93.pdf
Note that at if there are any tanks buried or partially buried in a basement, there are other requirements for decommissioning.