As urban self-organization grows into a key concept in spatial planning-explaining spontaneous spatial transformations-the understandings and applications of the concept divert. This article turns to the ontological dimension of urban self-organization and scrutinizes how a critical realist and a post-structuralist ontology inspire theoretical practices, analytical tendencies, empirical readings, and subsequent planning interventions in relation to urban self-organization. This is illustrated with an example of the self-organized regeneration of a deprived street in Rotterdam, the Netherlands. With this contribution, we aim to create ontological self-awareness among planning scholars in studying urban self-organization and invite them to reflect on how their positions complement, deviate, and potentially challenge or inspire those of others. We argue that by clarifying ontological diversity in urban self-organization, theoretical practices and complexity-informed planning interventions can be further deepened and enriched.