I've got a 4" angle grinder I was planning on using to cut it open, but I was wondering if it would ignite the oil. Anybody know or have a different way to cut it?

Thanks,
Scott

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.

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

Scott

... 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.

Not the fastest, but no chance of fire.

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.