If you're buying an old house in the Twin Cities with pipes sticking out of the ground in your yard, heads up. You might have an oil tank somewhere at the property. While the vast majority of homes in the Twin Cities are heated with natural gas, there is still a small percentage of homes in the metro area that are heated with fuel oil, and far more houses with abandoned oil tanks.
When a home gets converted from fuel oil to natural gas, the fuel oil tank becomes abandoned. Once the tank is abandoned, it needs to be dealt with.
When tanks are abandoned
If the tank is buried, it needs to be removed or filled in place If a fuel oil tank is left buried, it could eventually leak. A leaking underground storage tank (LUST) can contaminate the soil as well as the home, creating an environmental hazard that can cost a ridiculous amount of money to clean up. You can visit the EPA's web site on LUSTs for more info.

When a tank is located inside the house but not buried, it needs to be properly disconnected, and sometimes removed. A fuel oil tank takes up a large amount of room, so most people choose to have them removed, but requirements vary from city to city. For example, once a fuel oil tank is abandoned in Minneapolis, it needs to be removed from the property. This is written in to their Truth-In-Sale of Housing Evaluator Guidelines under item #25. The guidelines state:
Another option for an abandoned fuel oil tank is to stick it out in your front yard and paint it like a cow. You might think I'm kidding, but I've seen it done several times.

Clues to a buried fuel oil tank
The easiest way to identify a potential buried fuel oil tank is to look for a fill pipe and vent pipe at the exterior of the home. Sometimes the pipes will go through the foundation wall of the home.

Sometimes they just go down in to the ground.

When fuel oil tanks are removed, the fill and vent pipes need to be removed or cut off and filled with concrete. If you find pipes sticking out of the ground or foundation wall like the ones shown above, it probably means one of two things: either the tank is still there, or it was removed by a hack. No professional oil tank removal contractor is going to leave the vent and fill pipes looking like that.