What is oil field service?

"WHAT IS AN OILFIELD SERVICE COMPANY? You think of oilfield service companies as those who do the grunt work in the oilfield. Officially, oilfield equipment and services (OFS) is all the products and services that help with oil and gas exploration and production. It includes those companies that discover pockets of oil, energy data management, drilling companies, well construction companies, and production and completion services. These are the companies, like Northern Oilfield Services based out of Williston, that repair and maintain oilfield equipment, as well as companies that are involved in the extraction and transportation side."

https://www.nos-llc.com/oilfield-equipment/what-is-an-oilfield-service-company/