Ocean observing is executed and financed by nations according to their national interest and needs, but aligned with European and international agreements and directives. ObsSea4Clim brings together key European and international actors within ocean observing science, climate assessment, Earth System modelling, data sharing and standards, with users of oceanographic products and services with the goals of: - improving sustained and multipurpose observations vital to European and global climate requirements, and - delivering an improved observation framework based on EOV/ECVs and embedded in a Rolling Review of Requirements approach. ObsSea4Clim will: - improve regional and global climate assessments and - provide projections and actionable indicators for sustainable development provided. The work program is organized around four building blocks (TRL7-8): 1- the EOV/ECV concept, 2- the regional ocean indicator concepts, 3- the nations’ multi-purpose (climate, services, ocean health) ocean observing, 4- the integration with European and global initiatives. ObsSea4Clim will make use of ocean climate application areas such as: sea ice loss, ocean transports, stratification, sea level, ocean warming and marine heat waves and ocean mesoscale. Regional observing will be transformed into an EOV/ECVs framework suitable for regional and global climate products, ESM development (CMIP7) and indicators. ObsSea4Clim will also link to the Mission Restore our Ocean 2030 lighthouse basins. AI methods will control data quality 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, ObsSea4Clim will ensure sustained EU leadership in the ocean–climate–biodiversity science nexus at a global level.