In the light of an educational emergency aggravated by the health emergency following the COVID-19 pandemic, the DELTA (Digital Education for Learning and Teaching Advances) Research Group developed four open online courses available to lower secondary school students and teachers. The four courses, devoted respectively to Numbers, Space and Shapes, Data and Predictions, Relations and Functions, contain interactive materials to support the teaching and learning of Mathematics. We have made the four open online courses available to teachers, tutors, and students within two projects. Both projects are aimed at lower secondary school students, especially those with difficulties in Mathematics. In this paper we discuss the methodologies and technologies used to develop the online courses and we show an example of the interactive materials that can be found within the courses. The results show the appreciation of the resources by teachers, tutors and students, a sign that open online educational resources can foster a change in Mathematics teaching and learning. [For the full proceedings, see ED621108.]