The COVID Social Mobility & Opportunities study (COSMO) is a comprehensive national cohort study involving over 12,000 young people in England who were in Year 11 during the 2020-21 academic year. The study aims to assess the impacts of the COVID-19 pandemic on educational inequality and social mobility, focusing on how the pandemic has influenced educational trajectories and outcomes, particularly among disadvantaged groups. COSMO is a collaborative effort between the UCL Centre for Education Policy & Equalising Opportunities (CEPEO), the Sutton Trust, and the Centre for Longitudinal Studies (CLS). The study is funded by UK Research and Innovation and the Economic and Social Research Council, with fieldwork conducted by Verian. Data collection has occurred in two waves, with Wave 1 beginning in 2021 and Wave 2 in 2022. The study employs an area-stratified random probability sample from the National Pupil Database, with additional sampling from independent schools. It oversamples disadvantaged and minority groups to ensure diverse representation. Initial findings indicate significant challenges faced by young people, particularly those from disadvantaged backgrounds, in accessing mental health support and navigating educational disruptions. The study's data is available through the UK Data Service, and a potential Wave 3 is planned, contingent on funding. COSMO's findings are crucial for understanding the pandemic's differential effects on young people's education and social mobility, providing insights for policymakers and educators to address these inequalities.