OER Knowledge CloudJournal ArticleWang, XiaochenChen, TingtingZhang, YihanYang, Harrison HaoTo better understand the sustainable development of open educational resources (OER), this paper aimed to break through the original ‘fixed’, highly structured evaluation system and develop an open and flexible “1 + X” evaluation framework of OER by using the modified Delphi method. “1” refers to the mandatory basic criteria that the repository must reflect essential attributes and core functions, which are consist of four components: information content, learning process, system technology, and user experience. “X” refers to optional distinctive criteria that the repository shall focus on one or more developmental orientations, which may include: resource evolution, international collaboration, inclusiveness and equality, open educational practices, and value-added services. Optional “X” criteria, in line with the criteria’s diversity aims, can lead to the sustainable development of OER and globalized lifelong learning. The framework of this study provides a basis for the evaluation and optimization of existing OER repositories, and also serves as functional scaffolding for the construction of new repositories.20212021/02/04EnglishSpringer NatureEducation and Information Technologies2021China10.1007/s10639-021-10452-zyesopen educational resourcesrepositoryevaluation criteriaDelphi methodsustainable developmentBeijing, China, Asia