Bottled Oil is very important in the game, and you will most likely need hundreds, if not thousands of it during your late game progression. This is because you need a lot of Bottled Oil to produce plastic, which is one of the main crafting component for the end-game.
Getting the Off-Shore Drill required to make Bottled OilThe key to getting Bottled Oil in Forager is to unlock the skill called "Drilling". This skill is around the left top corner of the skill tree, and once you have this, you can create a building called the "Off-Shore Drill". The cost 5 x Steel and 20 x Brick to produce, and you will probably only need a few of them, depending on how serious you are about maximizing your production of Bottled Oil.
The Off-Shore Drill will need to be placed on an empty 5x5 grid in water, so you might need to search a bit before finding a suitable location. Now just build it like and building, and it's ready to start production.
Making Bottled Oil with the Off-Shore DrillWhen the Off-Shore Drill is ready to go, beging to produce Bottled Oil by using 1x Bottle to create 1x Bottled Oil, so make sure to keep producing a steady amount of Bottled to fill up with oil. There is not need for other materials, and all the building does is to fill the bottle with oil really.
It takes a little while to produce even a single Bottled Oil, so you might need to get 2-3 Off-Shore Drills if you want to make Plastic at a decent pace.