I'm pretty pro-market, but Standard Oil used a lot of techniques to fix prices. Having a lot of refinery control and the ability to undercut anyone who didn't play ball with them (and thus kill the small independent) did happen. They didn't need a true monopoly, just enough power to tank any smaller company that tried to undercut them.

If you're interested in the subject, The Prize: The Epic Quest for Oil, Money, and Power by Daniel Yergin is a fascinating read.