As an action, splash onto target within 5’, or throw up to 20’, shattering it on impact.
Make a ranged attack, treating the flask as an improvised weapon. On a hit, the target is covered in oil. If the target takes fire damage before the oil dries (after 1 minute), the target takes an additional 5 fire damage from the burning oil.
Alternatively, as an action you can pour the oil into a 5 foot square of level ground. If lit, the oil burns for rounds and deals 5 damage to any creature that enters or ends their turn in the area. A creature can take this damage only once per turn.