He took out all of the other oil industries and made no competition. He was the only oil industry.
Example: If there is gas station A and B on the same street. Gas station A sells gas for $1 and B sells for $2. Everyone goes to A and B cannot buy any more gas because they get no customers/sales. Gas station A takes out all of the other gas stations and then raises the price to $2 and everyone has to buy it from A.