We introduce the Opposition Movements and Groups (OMG) dataset, which includes rich information on 1,456 mass mobilization movements (and 1,805 movement-phases) across the world between 1789 and 2020. The stated goals, duration, size, tactics, ideology, and social and organizational composition of these movements have influenced political and social life worldwide. The OMG dataset includes more than 100 variables to capture these features, across countries and over time, thereby providing a key ingredient for systematically describing and understanding politics and societies in the modern era. We discuss our motivation for collecting this new dataset, before describing relevant existing datasets and how OMG complements them. We thereafter detail OMG’s contents, how it’s been constructed, and its validity and reliability characteristics. We showcase the data by first documenting the relative frequency of (predominantly) violent and nonviolent movements back to 1789. Second, we display and discuss how the stated ideologies of campaigns have shifted through time. Third, we map dominating social groups in the mass movements across major geographical regions and time. Fourth, we consider trends in mass movement organization.