OneArgo will transform the revolutionary 'core' Argo array (which tracks the upper ocean physical state) to one that has truly global reach, including the polar oceans and marginal seas, extending to the full ocean depth and including ocean biogeochemical measurements. Through Argo's novel data management system, all data will be freely shared in real-time with a very high quality version delivered within 12 months. Implementing OneArgo will greatly increase Argo's already remarkable impact on ocean and climate services, predictions and research, and enable groundbreaking developments in understanding ocean ecosystems, forecasting ocean productivity and constraining the global carbon and energy budgets.