The opportunities and benefits of collaboration within Diamond OA publishing are evident on individual, regional, national and global levels. Expanding beyond the confines of individual efforts offers significant benefits for enhancing publishing efficiency, quality, and impact. This approach also paves the way for fostering more productive collaborative opportunities in the future. In its effort to strengthen the Diamond OA archipelago and ensure its future financial sustainability, the DIAMAS project has aimed to identify current collaboration patterns at various levels, the obstacles hindering such collaborations, and potential new methods for publishers and service providers to work together. This study is based on a review of DIAMAS web survey responses, DIAMAS-led consultations with different stakeholder groups through public webinars and events, and six dedicated focus groups held with institutional publishers from across Europe in multiple languages, and a bibliometric investigation into Diamond OA journals and publishers. The document is split into three main sections: Unlocking the potential of collaboration in Diamond OA publishing Showcases evidence for the current level and nature of collaboration between Diamond OA publishers in Europe. Presents findings stemming from Diamond OA publishers in Europe on their perspectives and experiences with collaboration, structured around key publishing functions. Obstacles for collaboration in Diamond OA publishing (and how to overcome them) Identifies obstacles for various types of collaboration in Diamond OA publishing and, when possible, documents how these challenges have been overcome. This is organised by obstacle type. Collaboration patterns for European Diamond OA publishing Offers a bibliometric analysis of collaboration between institutional and professional publishers for Diamond OA in Europe.