This document is the official introductory brochure for OceanSOS, a 4-year Horizon Europe research project running from May 2026 to April 2030. OceanSOS is a transdisciplinary consortium combining basin-scale ocean modelling, deep-sea research expeditions, and international policy expertise to safeguard deep and open ocean ecosystems in an era of climate change and emerging anthropogenic threats. The brochure outlines the project's core mission to create a safe operating space for the deep and open ocean, its 5 strategic objectives, 7 interconnected work packages, 3 key high-seas study areas (NACES MPA, Agulhas Current System, and Brazil-Malvinas Confluence Zone), and its public outreach initiatives, including a full-dome digital planetarium show and the SEAmester Floating University.