Oleum is an unincorporated community

Unincorporated area

In law, an unincorporated area is a region of land that is not a part of any municipality.To "incorporate" in this context means to form a municipal corporation, a city, town, or village with its own government. An unincorporated community is usually not subject to or taxed by a municipal government...

 in Contra Costa County

Contra Costa County, California

Contra Costa County is a primarily suburban county in the San Francisco Bay Area of the U.S. state of California. As of the 2010 census, it had a population of 1,049,025...

, California

California

California is a state located on the West Coast of the United States. It is by far the most populous U.S. state, and the third-largest by land area...

. It is located on the Southern Pacific Railroad

Southern Pacific Railroad

The Southern Pacific Transportation Company , earlier Southern Pacific Railroad and Southern Pacific Company, and usually simply called the Southern Pacific or Espee, was an American railroad....

 6 miles (10 km) east-southeast of Point Pinole, at an elevation of 203 feet (62 m). An oil refinery is located at Oleum, hence the name from "petroleum".

Oleum is located on the Mexican land grant Rancho El Pinole

Rancho El Pinole

Rancho El Pinole was a Mexican land grant in present day Contra Costa County, California given in 1842 by Governor Juan Alvarado to Ygnacio Martinez...

 made to Ygnacio Martinez. A post office operated at Oleum from 1910 to 1951.