Any way to clean an abandoned oil storage tank-use as rain barrel?

You did not mention if it was steel or plastic. Seems like it's worth a try to use for agricultural water.

http://www.tractorbynet.com/forums/oil-fuel-lubricants/139410-how-do-i-clean-diesel.html

https://www.navitron.org.uk/forum/index.php?topic=12803.0

The cleaning chemicals are probably as toxic as the old barrel - wouldn't even water plants from it or turn it into a grill.

I doubt that you would be able to really thoroughly clean it out. Over time the fuel oil permeates into the metal. You can try to clean it out as much as you can with degreasers and solvents. Then you can have it lined with rubber coating on the inside. Similar as the way automobile gas tanks are cleaned, de rusted, and vulcanized inside and out with rubber.

Nope too toxic and the bottom is probably pure sludge, that is illegal to dispose of in landfill. Just buy a nice blue 50 gal rain barrel.

Unfortunately there is no safe way to get all the old, toxic chemicals out of an old oil barrel. I know its a terrible waste, but it wouldn't be safe to drink or use on plants.

Don't use that for anything you might ingest or water with. Too toxic!

Bummer - what a shame.