Open Innovation has been one of the most‐debated topics in management research in the last decade. Although our understanding of this management paradigm has significantly improved over the last few years, a number of important questions are still unanswered. In particular, an issue that deserves further attention is the anatomy of the organizational change process through which a firm evolves from being a Closed to an Open Innovator. The paper represents a first step in overcoming this limitation. In particular, adopting a longitudinal, firm‐level perspective, it addresses the following question: which changes in a firm's organizational structures and management systems does the shift from Closed to Open Innovation entail? In answering this question, the paper uses established concepts in organizational change research to look into a rich empirical basis that documents the adoption of Open Innovation by four Italian firms operating in mature, asset‐intensive industries. The results show that the journey from Closed to Open Innovation involves four main dimensions of the firm's organization, i.e. inter‐organizational networks, organizational structures, evaluation processes and knowledge management systems, along which change could be managed and stimulated.