United States v. O'Hagan is a landmark Supreme Court case decided in 1997 that addressed insider trading and established the 'misappropriation theory.' The case involved James O'Hagan, a lawyer who traded on confidential information about a merger while working for a law firm representing the company involved. This ruling clarified that individuals could be held liable for insider trading if they misappropriate confidential information, even if they are not corporate insiders.