The implementation phase of the energy system transition has shown that ambitious decarbonisation strategies must not only encompass radical techno-economic change but also incorporate societal and political dimensions as well. Socio-Technical Energy Transitions (STET) represents the cutting-edge of truly interdisciplinary academic research - incorporating a marriage of qualitative and quantitative elements in the multi-level perspective, co-evolutionary theories, the application of complexity science, and the use of adaptive policy pathways. However despite the vibrancy of academic research, the impact of STET research on policy and industrial decision-making to date has been negligible. This proposal (O-STET) is focused on operationalising and applying this highly novel interdisciplinary approach. O-STET will have four main concrete deliverables via two contrasting approaches: A. STET modelling 1a An open-source modelling framework with agent specific decision-making, and positive/negative feedbacks between political and societal drivers. 2a A stripped down decision maker tool for iterative stakeholder engagement. B. STET scenarios 1b Logically consistent, uncertainty-exploring scenarios, to frame both qualitative dialogues and existing energy models. 2b In-depth perspectives on branching points and critical components. The proposal team combines the UK's leading energy systems modelling group (at UCL) with the UK's leading innovation and transitions group at the University of Sussex. The PI is highly experienced at leading major whole systems projects with deep interaction with key stakeholders. In this he is closely supported by the Co-Is at Sussex and UCL, all of whom have a demonstrable success in collaboration, management and output delivery on past EPSRC projects. Responding directly to the requirements of this EPSRC Call, the O-STET project is structurally embedded with the Energy Systems Catapult, acting as an external "Analytical Laboratory" to the ESC. O-STET will first provide a theoretical and research framing of the ESC's portfolio of energy models and wider project-based assets. Second, bilateral interaction with the ESC will enable novel STET modelling and scenario tools to be iteratively developed and operationalised. Third, to maximise the applicability of the outputs of these new perspectives we will produce a stripped down STET decision-maker tool with a clear graphical user interface (GUI), as well as in-depth perspectives on branching points and critical components for key elements of STET scenarios (for example, new business models). The O-STET project team and the ESC will then combine as a "Platform" to disseminate STET insights to the full policy and industry energy community, anchored through a set of 6 stakeholder and technical workshops. O-STET will have a major online presence where we will curate and disseminate the open source resources produced under the project; including full models, modular components for hybridisation with other models, model documentation, datasets, socio-technical modelling protocols, scenario templates, data, and policy briefs.