Since the launch of Blue-Cloud 2026, the project team has proactively worked on establishing strong partnerships and synergies with other projects and initiatives in the Open Science and DTO environments, aiming to create long-lasting relationships to attain the Blue-Cloud's long-term objectives. The team strongly believes that through collaboration and cooperation with initiatives operating in similar fields, the value of Blue Cloud services increases. As output of this effort to cluster with related projects and initiatives in Europe and beyond, the first version of the Blue-Cloud Stakeholder engagement and synergies report presents an overview of all the projects Blue-Cloud 2026 has established cooperations within the first two years (M24). The dialogues established so far represent the ground layer for collaborations to be pursued in the second half of the project's lifetime, leveraging on the public availability of the Blue-Cloud services (VRE, VLabs, WBs) and the wealth of data resources of Blue-Cloud. This document represents a first release of the Blue-Cloud Stakeholder engagement and synergies and a second release (D6.5) will be produced by M40. The document is structured as follows: Chapter 1 provides a general introduction and outlines the objectives of the deliverable. Chapter 2 gives an overview of Blue-Cloud's role in the open science and DTO environments. Chapter 3 explains the approach used to monitor the status of established synergies and identify new ones. Chapter 4 details the synergies established within the EOSC remit. Chapter 5 focuses on the synergies formed under the Digital Twin of the Ocean initiative. Chapter 6 highlights the collaborations fostered during the 1st Blue-Cloud Federation Workshop. Chapter 7 outlines synergies established with other projects and initiatives. Chapter 8 draws the conclusions and summarises the next steps.