Ocean observing is executed and financed by nations according to their national interest and needs, but aligned with European and international agreements and directives. To improve sustained and multipurpose observations vital to European and global climate requirements, ObsSea4Clim brings together key European actors within ocean observing science, climate assessment, Earth System modelling, data sharing and standards, with users of oceanographic products and services to deliver an improved observation framework based on EOV/ECVs and embedded in a Rolling Review of Requirements approach. This way, regional and global climate assessments will be improved and projections and actionable indicators for sustainable development provided. The work program is organized around four building blocks (TRL7-8) including: the EOV/ECV concept, regional ocean indicator concepts, the nations’ multi-purpose (climate, services, ocean health) ocean observing and integration with European and global initiatives. Making use of ocean climate application areas (sea ice loss, ocean transports, stratification, sea level, ocean warming and marine heat waves and ocean mesoscale), regional observing is transformed into an EOV/ECVs framework suitable for regional and global climate products, ESM development (CMIP7) and indicators. This will also involve linking to lighthouse basins of the Mission Restore our Ocean 2030. AI methods will provide data quality control and unlock EOV/ECVs from existing data repositories. ObsSea4Clim will promote the implementation of FAIR/CARE data, enabling and advocating their use for versatile applications and referenced to an EOV/ECV framework. ObsSea4Clim will actively lead the dialogue with European and international climate and data initiatives to ensure the necessary exchanges for implementing the ObsSea4Clim results into the international system. In this way, we ensure sustained EU leadership in ocean–climate–biodiversity science nexus at global level.