There’s not going to be a moment when the world suddenly goes from having oil to having no oil. There are oil reserves that are relatively cheap and easy to extract, and there are other, very large reserves that are currently so difficult and expensive to extract that doing so isn’t profitable. As the easy oil gradually runs out, the supply drops, the price rises, and sources of oil that were not profitable at the old price become profitable. This maintains the supply of oil and stabilizes the price.

It’s true that the amount of oil is in principle finite, but that limitation isn’t really relevant on the timescales we’re talking about.