What I want to know is how do I find out if the oil tank was removed/decommissioned properly?

My question is can you decommission one unit of a five unit building in order to secure conventional financing?

Maybe you can find a 5 unit, and "decommission" one of the units, so you can get cheaper residential financing, and live in one of the 4, until you can occupy the 5th unit.

The third tank (a 10,000 gal tank) was left behind, and there was no documentation stating that it was officially "decommissioned".

Sometimes they do a condemnation report on burnt properties.Of concern:  Make sure that your purchase offer is clear on any rental obligations, that they are paid up to date, if there is an end of rental contract, if there is a decommission contract. 

If there is an underground oil tank, you will need to address it by either # 4 or 5, if you don't have luck with # 3. 3- Check in the local building department if the previous owner took out permits to properly decommission the oil tank.

I do know that they decommissioned the battalion that I was in and maybe one another and that will have an effect on the population of course, it may have something to do with the prices and foreclosures as well.

Here in RI there was a serious bill passed to decommission and replace cesspools.

Decommission the oil tank or it will cause undue issues when you try to sell.

No I don’t think so if it has been formally decommissioned (forget the actual word).