In the post-pandemic world, UNESCO advocates the development of open educational resources (OER) to remedy the financial pressures of students and their families. We thus conducted a dual-cycle action research study aiming to develop a sustainable model with minimal cost for creating OER for secondary school mathematics teachers and students. Our theoretical foundations drew upon existing sustainable models of OER development and experiential learning theory to enhance the efficacy of OER. Formative evaluation techniques were employed in both action research cycles, where pre-service teachers developed the OER and in-service teachers provided suggestions for improvement. In the second cycle, we tested the use of our OER with 35 Grade 11 students to examine both student achievement and retention. The test results showed a significant learning gain (pre-test: Mdn = 2.00; post-test: Mdn = 10.00), which was retained over 2 weeks (delayed post-test: Mdn = 11.00). However, the students' performance on more advanced questions was not satisfactory. Based on an overall reflection on the study, we proposed a sustainable model of OER development, which leveraged the manpower of pre-service and in-service teachers and incorporated formative evaluation techniques. Furthermore, we provided recommendations for enhancing the future development of OER in mathematics education.