Open Science advocates for the democratisation of scientific research and its outputs, emphasising accessibility, transparency, and collaboration. This movement aims to enhance the quality, efficiency, and impact of scientific work, thereby fostering increased public trust and broader engagement. Central to this ecosystem is the Honest Broker, an entity (i.e. a person or a group of people) that plays a crucial role in bridging the gap between scientific research and policy-making. Honest Brokers act as neutral intermediaries, facilitating the use of scientific evidence in decision-making without pushing a particular agenda. They ensure that policy and decision-makers have access to reliable, authoritative, relevant scientific evidence, helping them to navigate the complexities of the Open Science resources. Open Science poses unique challenges and opportunities at the interface of science and policy. Access to vast amounts of scientific information can be overwhelming; hence, the Honest Broker’s role is crucial in making this information understandable and actionable. Honest Brokers do not limit the options available to decision-makers but instead clarify and expand the scope of choice for decision making, allowing for decision makers to reduce choices based on their individual values and preferences. This role requires a deep understanding of both scientific content and the policymaking process, ensuring that the advice provided is both relevant and timely. This document tries to provide a comprehensive set of guidelines and best practices for Honest Brokers operating within the Open Science ecosystem. By detailing these roles and practices, the document not only aids Honest Brokers but also encourages researchers to embrace Open Science, enhancing the overall integrity and impact of scientific endeavours in public decision-making.