The oil cartel sets the price. You're getting wrapped around the word "demand." I don't mean demand for the product; I mean the price demanded FOR the product.
Firt of all, cartels don't set prices, they influence them by influencing supply. It is true that OPEC is able to influence prices by influencing supply. However, all apperences are that everyone is producing and selling as fast as they can in order to take advantage of the high prices.