During the Texas oil boom years in the first half of the 20th century, what groups were

not hired by the oil companies to work in the oil fields?

During the Texas oil boom one would find that there were few locals involved in the work.

Explanation:

Most of the businessmen who profited from the oil boom were not from Texas.

They were outsiders who often brought their own field workers and specially their own engineers to work the field and the natives were at best given menial jobs at the time.

Texas had been a largely agrarian economy until the oil boom and most of the population was involved in farming activities so it was only seen as proper that the farmers did not know how to work these areas and were not given the jobs.