I always threw away the oil cap: a colleague explained to me what it was really for.

A tab with a double role
The tab plays an important role in the oil preservation process. It acts as an airtight seal, preventing air from getting inside the bottle. Air can be harmful to oil, as it can accelerate its oxidation and alter its flavor. It ensures better preservation of the oil and extends its shelf life.
Controlling the amount of oil to use
When you want to prepare a recipe that only requires a drizzle of oil, it is sometimes difficult to control the amount that flows from the bottle. As a result, you always end up putting more than you need. But thanks to the plastic tab inside the bottle, under the cap, this problem will be quickly solved.
Indeed, the small ring allows you to partially block the spout of the bottle, and therefore, to more easily control the amount of oil to pour. This way, you can pour this liquid without losing a single drop. Pretty practical, right!?

However, be careful not to press too hard on the tab so as not to push it inside the bottle. To do this, simply press gently and lightly.