Severe hydro-meteorological phenomena are having a high impact in European territories and are of global concern. The science behind these phenomena is complex and advancement in knowledge proceeds with progress in data acquisition and forecasting useful for real-scenario interventions. The employment of nature-based solutions (NBS) to mitigate the impact of hydro-meteorological phenomena is not adequately demonstrated, still uncoordinated at the European level, therefore not reaching full potential. Actions to achieve highest NBS impact requires strategies to enhance societal acceptance, policy strengthening while demonstrating advantages for market development. The objective of OPERANDUM is to reduce hydro-meteorological risks in European territories through co-designed, co-developed, deployed, tested and demonstrated innovative green and blue/grey/hybrid NBS, and push business exploitation. It aims provision of science-evidence for the usability of NBS, best practices for their design based on participatory processes. It foresees a multiple level of stakeholders engagement from the local community up to the international level to leverage widest possible NBS acceptance to promote its diffusion as a good practice. It establishes the framework for the strengthening of NBS-based policies according to local legislation and promotes technology and innovation in NBS to create a European leadership. OPERANDUM is based on open-air laboratories (OALs), a fairly new concept that expands the Living Labs to a wider vision for natural and rural areas. In OALs novel NBS in seven European countries and three in China and Australia are implemented to address specific risks and their effectiveness, assessed through innovative monitoring systems and cutting-edge numerical modelling approaches. OPERANDUM realizes a multi-dimensional open and flexible platform enabling stakeholders and end users to improve knowledge in NBS to mitigate climate change as well as ways to promote and exploit the improved/preserved environment while increasing business opportunities.