I belive heating oil is basically diesel and diesel isn't volatile like gasoline.
That bring said , if I wanted to cut a tank and was afraid of igniting the residual contents, I would fill the tank up with
water, and get to cutting.
If you are concerned about using electric tools so close to water, drain down to just below the cut line, and then get to cutting.
Even just a sprinkler set up inside the tank could mitigate the possibility of any ignition.
Instead of making char with the first burn, maybe use nice dry fuel and burn it hot and fast as possible.
You might want to protect the ground underneath by elevating the kiln on
concrete blocks or metal legs.
My own kiln is a 50 gallon water tank with a long notch cut out of one side, I do wish I had a lid for it.
If you can get the company to turn the branches into woodchips, I would do that AND cut more for making biochar in the 250 gallon retort.
The chips should work well in a TLUD charcoal retort.
TLUDs take less tending, and its easier to tap into their process heat for cooking, etc.