For teaching practice to be useful, educational materials must be thoroughly evaluated prior to use. In this study, teachers evaluated open educational resources (OER) created for a European Commission Horizon 2020 project called DIALLS ('DIalogue and Argumentation for cultural Literacy Learning in Schools'). Considering research on how OER are evaluated, we were interested in learning (1) how teachers perceive their "application," "content," "design," "value" and "sustainability" and (2) what other aspects teachers find important when evaluating OER. After analysing quantitative and qualitative data, we found that aspects facilitating self-regulated use were rated most highly and mentioned most often. Teachers additionally valued "adaptable" materials offering "opportunities to reflect" their teaching practices. Future research may benefit from evaluating OER based on 'affordance' aspects -- aspects that allow teachers to adapt the materials in a self-regulated way and reflect on their teaching.