We present a methodology to systematically assess the scale and impact of an organization’s contributions to open source software (OSS). The methodology combines the archival data of Software Heritage with usage metrics, dependency analysis, economic valuation models, and interviews to comprehensively understand institutional OSS involvement. We then apply the methodology to the European Organisation for Nuclear Physics (CERN). Despite using mostly commit data, we obtain a thorough overview of CERN’s OSS engagement. We identify over six million commits made to over 50,000 projects and highlight the most impactful projects led by CERN. Beyond CERN, the methodology offers a reusable framework for organizations seeking to measure and evaluate their OSS contributions.