Evidence-based Key Impact Pathways were validated through systematic literature reviews, stakeholder workshops, and empirical case studies. Three peer-reviewed papers explored academic, economic, and societal OS impacts, while a synthesis report translated findings into policy recommendations. The PathOS OS Impact Indicator Handbook introduced indicators for diverse OS practices with clear methodologies and datasets, forming a foundation for reproducible measurement. Indicators were refined through case study operationalisation and community sprints. Six case studies explored Open Access effects on climate-AI research persistence and gender equity, repository type influences on social sciences data reuse, COVID-19 data reusability impacts, RCAAP infrastructure’s industry visibility, ELIXIR bioinformatics benefits, and French OA platform reach beyond academia. These produced new causal evidence, reusable datasets, and tools. PathOS also developed a dedicated CBA framework, applying it to ELIXIR’s UNIPROT and RCAAP infrastructures using mixed quantitative and qualitative approaches. Training and engagement activities integrated methodologies into a coherent programme, accompanied by policy briefs, factsheets, and an Open Science resource hub. The project ensured strong coordination, quality assurance, and dissemination through effective management and advisory oversight.