If you have experiance as a motorman then it should not be difficult. Simply research the internet for a busy oil field town and search local jobs there.
If you do not have experiance it may take years to become a motorman. First, oil drilling companies rarely hire a green employee with no oil field experiance at all unless there in a busy town and desperate for help. Otherwise, you will have to start out working foor a service company that does hire green employees. After that you may get hired on as a floorhand. After that you simply must work hard and show aptitude until you eventually move up to a motorman.