Background <p>Patient and Public Involvement and Engagement (PPIE) is increasingly recognised as essential to the relevance, feasibility, and impact of paediatric intensive/critical care research. However, reporting of PPIE processes and outcomes, particularly in child health studies, remains limited, often lacking detail on sustained involvement and its influence across the research lifecycle.</p> Case study <p>This case study presents the PPIE approach embedded within the OCEANIC Study, a multi-centre longitudinal mixed-methods investigation into the outcomes and support needs of children and families following paediatric intensive care unit (PICU) discharge. The study uniquely sustained engagement with a Children, Young People and Parent Advisory Group (CYPPAG) over eight years, aligning with the National Institute for Health and Care Research (NIHR) standards for public involvement. We detail the strategies used to foster inclusive, respectful, and impactful collaboration, including co-development of study materials, iterative feedback on recruitment and retention strategies, and involvement in qualitative data interpretation and dissemination. The advisory group’s contributions shaped study design, enhanced recruitment, and informed dissemination, including co-authorship and conference presentations. PPIE impact was explored qualitatively through CYPPAG feedback, tracking of study changes informed by the CYPPAG, and ongoing reflection within the research team.</p> Conclusion <p>The OCEANIC Study demonstrates the feasibility and value of longitudinal, embedded PPIE in paediatric critical care research. It offers a novel model for sustained advisory group involvement, evidencing how meaningful collaboration can improve research relevance, rigour, and reach. This case study provides actionable insights for researchers seeking to integrate PPIE across the research continuum and highlights the therapeutic and empowering impact of involvement for contributors.</p>