A sleazy doctor working in Las Vegas, who was Kolchak's informant. He performed illegal operations in closets (possibly abortions) and was threatened with disbarment for malpractice. Kolchak used his illegal activities to blackmail him into giving him information; however, when he tried to get information on Janos Skorzeny’s murders, O’Brien refused to answer initiall, though he did inform Kolchak that the victim’s blood had been drained.