Oil trade was always a completely private enterprise and it changed when entering 20th century with market competition.
Step-by-step explanation:
Before the coming of the time of the non monopoly states with competition in business as their core value, their were monopolies over certain businesses of certain people.
Theodore Roosevelt passed laws against this.
This changed the industry meaning that a lot of businessmen small and big started spring up and localized their oil manufacture.
There were also some that were concentrating in making oil fields in the sea which also became very profitable in the future.