Final answer:
The major discovery of oil in the North Sea, specifically off the coast of Norway, has decreased the global demand for OPEC oil by providing an alternative source of oil.
Step-by-step explanation:
Major oil discoveries that have decreased demand for OPEC oil include the discovery of oil in the North Sea, particularly off the coast of Norway. This significant find put countries like the U.K. into the oil business, providing a non-OPEC source of oil to the global market. Other factors also play a role, such as the development of more fuel-efficient cars, which reduces overall oil demand, and the increase of alternative energy sources like solar power, which compete with oil as an energy source.