So, Oil is really not explosive. To get Oil to explode you would to have as a vapor mixed as oxygen under pressure. Oil Fire yes, exploding like it does in DOS:2 no. Also the barrels are stated to be 20 pounds which means they are 2.5 Gallon / 10 Liter Kegs, not the size you have in the game.
If you want the ex plosive kegs, use Smokepowder from the 5e rules.
Smokepowder
Wondrous Item, uncommon
Smokepowder is a magical explosive chiefly used to propel a bullet out of the barrel of a firearm. It is stored in airtight wooden kegs or tiny, waterproof leather packets. A packet contains enough smokepowder for five shots, and a keg holds enough smokepowder for five hundred shots.
If smokepowder is set on fire, dropped, or otherwise handled roughly, it explodes and deals fire damage to each creature or object within 20 feet of it: 1d6 for a packet, 9d6 for a keg. A successful DC 12 Dexterity saving throw halves the damage.
Casting dispel magic on smokepowder renders it permanently inert.
So you have a 20 pound Keg of Smokepowder that could be a 9d6 bomb.
Now it begs the question of why Smokepowder has become so common that it's just laying around everywhere.
Personally I hope they just remove the barrels, cheesy game play that not many human DM would allow at their table but Larian Studio is the GM so the rules are whatever they want.