The removal of oil, or any other contaminant, from an oil storage tank is a specialist function and correct disposal of the resulting waste material is controlled by environmental legislation.
The oil outlet is away from the bottom.
Some tanks do have a drain bung at other end .. if you are brave enough to drain a bit off!
Tanks are also installed so bottom edge is lower than the outlet end lower edge (ie .. out of level)
When it happened to us local companies wanted to charge a small fortune. We only discovered the water in the tank when the heating started to play up. I found this site http://www.serpro.eu/shop1/Water-Soakers.html and they worked brilliantly.