Like one poster wrote, it will always probably smell and taste like fuel oil, so I would suggest never using it to water animals.
But once it has been rinsed out real well, it probably would be OK for carrying water for plants. I think if it was mine, I would put the tank on my trailer, fill it about 2/3 full of water and add a bottle of Dawn dishwashing liquid to it. Then I would haul it around for a while to get all of the inside of the tank well covered by the detergent solution. I would drain the tank somewhere that wouldn"t matter much and then carefully rinse it out. It would probably be about as good as it ever could be.
Another thought would be to put the tank, with all the bungs open, on a fair sized burn pile. The heat would probably vaporize most of the remaining fuel and it would burn off in the fire. If there was much fuel still in the tank, that might not be the safest plan, as you might produce a rocket or a bomb. And after the tank had burned and you used it to store water, I would expect that it would rust out fairly fast.
I sure would not ever consider a former oil tank for storing drinking water. Good luck!