Who monopolized oil in US in 1880?

In the 1880s, John D. Rockefeller and his company, Standard Oil, monopolized the oil industry in the United States. Through aggressive business practices, including horizontal integration and the establishment of trusts, Standard Oil controlled a significant portion of oil production, refining, and distribution. By the end of the decade, it held a dominant position in the market, leading to widespread criticism and eventually regulatory reforms.