Open source software for nuclear reactor analysis is reviewed, highlighting innovations, challenges, collaborations, education, and training needs. Many institutions have invested significant resources in developing modern open source software for analysing nuclear reactors. Recognizing this trend, the IAEA has taken the initiative to streamline and support the Open-Source Nuclear Codes for Reactor Analysis (ONCORE) initiative. Within the framework of ONCORE, the IAEA organized a technical meeting, titled ""Development and Application of Open-source Modelling and Simulation Tools for Nuclear Reactors"", in 2022 in Milan, Italy. This publication serves as a comprehensive review of the current state of the field, featuring insights from experts, experiences and the latest innovations and technological challenges in open source software and open access data. It also highlights ongoing collaborations supporting research and development, as well as education and training in nuclear science and technology, drawing from the papers presented at the ONCORE meeting.