Background: Online postal self-sampling (OPSS) has been rapidly adopted as a method of sexual health service delivery in England, particularly since the COVID-19 pandemic. Access to this method of testing has increased whilst, concurrently, access to face-toface clinic services has decreased in many areas. OPSS now accounts for 43% of asymptomatic chlamydia testing among young people and is formally recommended by national guidance. There has been little research on the process of implementing OPSS and the contextual factors which affect its impact and sustainability. Methods: This qualitative study of implementation is nested within the ASSIST study, a mixed-methods, realist evaluation of OPSS with a focus on services in three key areas of England: Birmingham; London; and Sheffield. Semi-structured interviews, informed by Normalisation Process Theory (NPT), were undertaken with a diverse range of staff and stakeholders on their views and experiences of OPSS. Data were analysed using a realist logic of analysis, informed by concepts from NPT. Interviews were supplemented with an analysis of publicly available documents and those provided by services, organised to provide insights on implementation context. Results: Interviews have been conducted with 42 staff and 13 other stakeholders to date, with 57 documents sourced. We found that pre-existing organisational relationships influenced how staff perceived the value of OPSS and their buy-in to the process of introducing it. These relationships were less important to the implementation and sustained use of OPSS than commissioning arrangements -- which varied by case study area -- and wider service changes during and after COVID-19 lockdowns. The relationships between staff and stakeholders had a substantial impact on the success of implementation, in both the short- and long-term. Implementation was found to be a continuous process which needs to respond to changes in context, even years after services have launched. Differences in commissioning and delivery models affected the impact of COVID-19; it advanced implementation in some services and reversed it in others. Conclusion: The contextual factors identified, and their impact on implementation, will aid providers seeking to set up, or maintain, an OPSS service. Future research should explore implementation in countries and regions with different commissioning contexts and delivery models. [ABSTRACT FROM AUTHOR]