Oil is a very precious resource in Forager as it is the source of almost all of the complex and high tier crafting components. Items like plastic are made entirely of oil and since plastic is used in everything a factory produces, gathering oil should be something that you focus on as soon as you can.
Where to find oil
Oil can only be found by building an offshore oil rig. These rigs need to be built on water. You will need to walk out to it and provide empty glass bottles. After a fairly long wait, a bottle of oil will be produced by the oil rig. You can then use this bottle of oil to produce plastic and other items at a factory.
Due to the amount of time it takes to produce a bottle of oil, it is best to build a small farm of oil rigs with a nearby forge for producing glass bottles. In the later stages of the game, you will need to acquire a lot of oil and will need to do all you can to make it quick and efficient.