Oil City is the largest city in Venango County, Pennsylvania, United States. Known for its prominence in the initial exploration and development o…Oil City is the largest city in Venango County, Pennsylvania, United States. Known for its prominence in the initial exploration and development of the petroleum industry, it is located at a bend in the Allegheny River at the mouth of Oil Creek. The population was 9,608 at the 2020 census, and it is the principal city of the Oil City micropolitan area.