The increasing call for harm-focused approaches warrants a method for assessing the harm of organized crime. A harm assessment of organized crime can be of use to law enforcement agencies for prioritizing efforts and designing policies that aim to counter organized crimes based on the harm they cause. It can also be used by other institutions that can intervene and support law enforcement indirectly in preventing and counteracting organized crime. However, assessing the harm of organized crime entails dealing with the complex structure of intertwined criminal activities and addressing the fact that organized crime groups commit other crimes to support their main criminal activity. This research introduces a novel method for Organized Crime Harm Assessment (OCHA method) and demonstrates it with the application to the case of heroin trade into and within England and Wales. The challenges of applying the OCHA method are identified and discussed, as well as its limitations, contributions, and opportunities for future research.