Larian Studios Forums Divinity - Original Sin Divinity Original Sin - Help/Tips/Tricks Summation of the Spell Combo Possibilities

Fire + Oil = Smoke Cloud over burning ground
Fire + Ooze = Explosion that leads to a Smoke Cloud over burning ground

The important thing is that it is these combos that result in the Smoke Clouds that hinder Line of Sight. Burning Ground on its own (without Oil or Ooze) gives off no Smoke.