This study aims to validate the equivalence of a comprehensive and condensed rubric for evaluating the quality of Open Educational Resources (OERs) in mathematics which are freely accessible and openly licensed educational materials and have gained widespread adoption due to their potential to enhance learning outcomes and increase access to quality education. However, the proliferation of OERs necessitates robust quality assessment methodologies to ensure their effectiveness and reliability. This study addresses this need by presenting a comprehensive assessment framework developed by the Erasmus + project Gate2Math, designed to provide insight into the systematic evaluation of OERs across various areas. The article reports on an empirical test of an OER quality assessment model developed within this project. Validation methods include analysis of variance (ANOVA) and reliability analysis, which are employed to ensure the rigor and dependability of the assessment model. By promoting a systems approach to OER assessment, the article seeks to support mathematics educators in making informed decisions and promoting equitable access to high-quality educational resources. The development and validation of such assessment methodologies are crucial for maintaining the standards and enhancing the impact of OERs in the educational landscape.