The more formal term for the type of firm usually referred to as a major oil company.
An integrated oil company is one engaged in all phases of petroleum operations, from finding the oil to selling petroleum products at retail outlets. Such a company is involved in exploration, production (drilling wells), transportation (moving product), refining (converting crude) and marketing (selling refined products through retail stations and aircraft fueling systems).
In a large company, all of these areas work together. The company is thus said to be integrated.