This paper examines an important, but hitherto relatively neglected, aspect of the work of Nobel Laureate Elinor Ostrom, namely her ontological commitments (that is, her views about the nature of social reality and their implications). While they are rarely described as such, Ostrom's writings contain discussions of many important ontological issues (including the openness of the social world, the nature of human agency, the configural nature of social rules; the relationship between social structure and human agency; and the complexity of the social world, viewed as a nested hierarchy of emergent systems). This article is the first to draw together systematically Ostrom's reflections about ontological issues and to identify what Ostrom saw as their implications for the appropriate methods for analysing and explaining the possibility of self-governance. In doing so, the paper also contributes to the growing literature on ontology and the history of economic thought.