The adoption of Open Educational Resources (OER) can, on the one hand, increase access and quality in higher education, but on the other hand it is raising concerns among universities and researchers about its economic sustainability. This is mainly because, unlike traditional online learning, in OER-based approaches learners do not have to pay to access learning resources, however the institution incurs costs for the production, maintenance and dissemination of OER. In this context, the United Nations Educational, Scientific and Cultural Organisation (UNESCO) has urgently called for more research on OER sustainability models in its 2019 OER recommendation. To contribute to a better understanding of this issue, this paper used the triangulation method to investigate the potential OER sustainability models that are currently implemented by universities, along with their challenges and possible developments. Through a comprehensive literature review and a 2-round Delphi method with thirty OER experts, ten OER sustainability models have been identified and analysed, where public and internal funding are the most established ones. The findings of this study could support organisations in developing their own OER sustainability strategy, facilitating OER adoption worldwide and therefore contributing to achieving the UN Sustainable Development Goals (SDGs).