Background: Stroke often results in chronic disability, with partners and family members taking on the role of informal caregiver. There is considerable uncertainty regarding how best to identify and address carers' needs. The Carer Support Needs Assessment Tool (CSNAT) is a carer-led approach to individualised assessment and support for caregiving that may be beneficial in palliative care contexts. CSNAT includes an implementation toolkit. Through collaboration, including with service users, we adapted CSNAT for stroke and for use in a UK stroke specialist organisation providing long-term support. The main aims of OSCARSS are to investigate the clinical and cost-effectiveness of CSNAT-Stroke relative to current practice. This paper focuses on the trial protocol, with the embedded process evaluation reported separately. Methods: Longitudinal, multi-site, pragmatic, cluster randomised controlled trial with a health economic analysis. Clusters are UK services randomised to CSNAT-Stroke intervention or usual care, stratified by size of service. Eligible carer participants are: adults aged > 18 years; able to communicate in English; referred to participating clusters; and seen face-to-face at least once by the provider, for support. The 'date seen' for initial support denotes the start of intervention (or control) and carers are referred to the research team after this for study recruitment. Primary outcome is caregiver strain (FACQ - Strain) at three months after 'date seen'. Secondary outcomes include: caregiver distress; positive caregiving appraisals (both FACQ subscales); Pound Carer Satisfaction with Services; mood (HADs); and health (EQ-5D5L) at three months. All outcomes are followed up at six months. Health economic analyses will use additional data on caregiver health service utilisation and informal care provision.Discussion: OSCARSS is open to recruitment at the time of article submission. Study findings will allow us to evaluate the clinical and cost-effectiveness of the CSNAT-Stroke intervention, directed at improving outcomes for informal carers of stroke survivors. Trial findings will be interpreted in the context of our embedded process evaluation including qualitative interviews with those who received and provided services as well as data on treatment fidelity. OSCARSS will contribute to knowledge of the unmet needs of informal stroke caregivers and inform future stroke service development.Trial Registration: ISRCTN Registry, ISRCTN58414120 . Registered on 26 July 2016.