PathOS aims to identify and quantify the Key Impact Pathways of Open Science relating to the research system and its interrelations with economic and societal actors. PathOS will enable a new understanding of OS impacts and their causal mechanisms through its workplan encompassing actions to synthesise and structure current evidence, development of new methods and tools for measuring impact, iterative pilot-testing via in-depth case studies, innovative dissemination and networking, and co-creation synthesis activities culminating in policy recommendations. This is pivotal in order to develop effective OS policy in the EU. It will do so by collecting concrete evidence of the causal effects of OS by studying the pathways of OS practices, from input to output, outcome and impact, including the consideration of enabling factors and key barriers. Impacts and pathways will be developed in particular in the three areas of science, society and economy. By investigating, measuring and comparing its costs and benefits together with its pathways, PathOS will (i) bring a better understanding of the implications of open science for science, economy and society, (ii) provide recommendations to policy makers and other actors in the R&I ecosystem as to how and to what extent open science should be promoted in a balanced way, and (iii) develop innovative tools and methods using a big data to augment traditional ones for studying the causal effects of open science. This will enable evidence-based Open Science policy prioritisation, maximum OS impact, and increased R&I capacity in EU research systems.