Oil is a fungible commodity. Virtually all oil is sold on the international commodities exchanges. Most oil is purchased by investors in the form of "futures." The investors, in turn, sell their oil to refineries. In most cases, you will never know exactly from where your gasoline originates.
Relatively little oil is actually "owned" by the oil companies, such as EXXON/Mobil, until they actually purchase it. They still own some properties in the USA and have the mineral rights to them. However, most oil originating in foreign countries is owned by those countries and they sell it in the open international commodities exchanges. OPEC is another animal. The members conspire to set base prices and the futures auction prices start there. They do this by reducing output, thus driving up the cost of all oil on the international market (remember, oil is a fungible asset).