Background: Patient, Carer and Public Involvement (PCPI) should be embedded in health care research. Delivering PCPI can be challenging, but even when PCPI is carried out it is rarely reported resulting in lost opportunities for learning. This paper aims to describe PCPI in the OSCARSS study, a pragmatic-cluster randomised controlled trial with an embedded economic and process evaluation. Methods: A carer research user group (RUG) co-developed OSCARSS to evaluate how to best deliver support to caregivers of stroke survivors. The PCPI activity involved regular meetings and preparatory work, from the initial conceptualisation of the study through to dissemination. Written reports, structured group discussions and individual interviews were carried out with the RUG and researchers to capture the added value and learning. This paper was co-authored by two of the RUG members with contributions from the wider RUG and researchers. Results: The core six members of the caregiver RUG attended the majority of the meetings alongside three researchers, one of whom was the co-chief investigator. PCPI was instrumental in changing many aspects of the research protocol, design and delivery and contributed to dissemination and sharing of good practice. There were challenges due to the emotional toll when PCPI members shared their stories and the extensive time commitment. Positive experiences of learning and fulfilment were reported by the individual researchers and PCPI members. Wider organisational administrative and financial support facilitated the PCPI. The researchers’ existing positive regard for PCPI and the clear focus of the group were key to the successful co-design of this research. Conclusions: The value and learning from the PCPI collaborative work with the researchers was of benefit to the study and the individuals involved. Specific PCPI influences were a challenge to pinpoint as successful co-design meant the researchers’ and carers’ contributions were intertwined and decision-making shared.