John D. Rockefeller was able to gain control over most of the oil industry by

He bought out other competitors as well as he also prevented rival companies from using the rail roads he held a controlling interest in.

Creating a monopoly. This means that he controlled the oil industry by buying out other companies and by controlling as much of the production of oil as he can. This lead to him being able to control the whole oil industry as no smaller industry for oil could open up