"Underground oil tank".

Presumably heating oil? #2? Referencing the fire triangle, you have one of the components, fuel.
You will provide another, source of ignition (angle grinder sparks).
The last, oxygen, is already in the tank.

You need to remove at least one of the components. So you drain all the liquid. There will be a film and saturated corrosion/debris products left.
The atmosphere can be purged with a plethora of methods; here's a few: several cups of baking soda. A line from your car's muffler(CO). The contents of a Co2 extinguisher. A purge using inert cylinder gas like nitrogen or Co2. Proprietary oxygen scavengers ala Purple K or Foray ( Ansul products). Mud pack. A method for smaller tanks is to almost fill with water- be advised that the water will be contaminated, don't do this unless you have a legit way to dispose of it.
I've always used a torch to chop up tanks after they're "safed off". The grinder will work.
#2 oil has a higher flash point for a flammable liquid and this isn't as scary as it may sound. Have a fire extinguisher handy.

Please be very careful. My BIL cousin purchased an old tank for something. He was cutting it open and was burned very badly, almost didn't survive. It was supposed to have been cleaned out professionally when he bought it, they knew what he was going to do with it and it was not cleaned out well enough. Big law suit, ruined lives.

I think you've posted this already in another part of gweb. Since you're even contemplating using an angle grinder, I would respectfully suggest it's something to give to a professional. By the time you pay to dispose of the oil and tank, you may as well pay a pro to remove it. It may be costly but for good reason, and it'll probably cost you almost as much.

Having said that, as I mentioned, I think you'd need someone with a waterjet cutting machine to do it. Take izzie's advice.

PJB999, with all respect, you'll never see a waterjet cutter used to chop up old tankage. It's mostly done with oxyfuel torches wielded by hand. I've never seen a hand held waterjet, how do you control 40,000PSI?
The principles and procedures described are hot work compatible with any of the big oil companies.

Thanks all. My worried wife called around and found a company that will take the tank for about $100, depending on how much residual oil there is.

Scott

A bargain, if you ask me.

The MUD PACK method sounds like a simple, safe, and ingeniously low-tech way to go. I'm ASS-u-ming the mud gets packed INSIDE the tank, and then you grind/saw through from the outside, the mud nicely insulating the sparks from the volatile atmosphere inside tank...

... just curious how you get the mud inside the tank... apologies in advance... FL-orida Boy, zero experience with Ye Olde #2 Heating Oil, and related infrastructure, lol.

I have put the tan on one wide, filled it with water with some soap (extend the inlet at right angles upwards above the top of the tank side) and then cut with a sawzall.

Not the fastest, but no chance of fire.

Water jet equipment is used to cut all sorts of sheet metal for ductwork.

Quick, not flames or heat, just water.

60,000 PSI is not uncommon.

It is not hard to handle (even by hand) since the mass of the water being ejected is very small.

Nozzles of 0.005 in diameter are not unheard of.

It is not a common for thick metal pieces since oxy-acetylene can make short work of them (though at some hazard for tanks that once contained flammable liquids without thorough cleaning).

Luckily fuel tanks are not normally all that thick walled.

I have welded holes shut in gas tanks up to 10,000 gallons 1/2 full. Lots of smaller tanks too. I cut up a couple 6,000 gallon diesel tanks for use around the place. The last cut and 0 2 rushes in and they POP. The trick is to upset the oxygen balance to below ignition. I do this by pumping exhaust fumes in for 15 or more minutes and leaving the vehicle running while cutting or welding. Do NOT try cutting through the end of a tank. If it does blow the end will peel out along with part of your head.