You can douse a creature, object, or space with Oil or use it as fuel, as detailed below.
Dousing a Creature or an Object. When you take the Attack action, you can replace one of your
attacks with throwing an Oil flask. Target one crea-ture or object within 20 feet of yourself. The target must succeed on a Dexterity saving throw (DC 8 plus your Dexterity modifier and Proficiency Bonus) or be covered in oil. If the target takes Fire damage before the oil dries (after 1 minute), the target takes an extra 5 Fire damage from burning oil.
Dousing a Space. You can take the Utilize action to pour an Oil flask on level ground to cover a 5-foot-square area within 5 feet of yourself. If lit,
the oil burns until the end of the turn 2 rounds from when the oil was lit (or 12 seconds) and deals 5 Fire damage to any creature that enters the area or ends its turn there. A creature can take this damage only once per turn.
Fuel. Oil serves as fuel for Lamps and Lanterns. Once lit, a flask of Oil burns for 6 hours in a Lamp or Lantern. That duration doesn’t need to be consecutive; you can extinguish the burning Oil (as a Utilize action) and rekindle it again until it has burned for a total of 6 hours.