I always throw away the oil cap: A colleague explained to me what it is actually for

The tab plays an important role in oil preservation. It acts as an airtight seal and prevents air from entering the inside of the bottle. Air can be harmful to oil as it can accelerate its oxidation and alter its taste. It ensures better preservation of the oil and extends its life.

Check the amount of oil to use
When you want to prepare a recipe that requires just a dash of oil, sometimes it is difficult to control the amount that comes out of the bottle. This means that we always end up using more than we need. But thanks to the plastic tab inside the bottle, below the cap, this problem is quickly solved.
In fact, the small ring allows you to partially block the bottle's spout and thus more easily control the amount of oil to be poured out. This way we can pour out this liquid without losing a single drop. Pretty handy, right?!?