The design and use of online materials for blended learning have been in the spotlight of educational development over the last decade. With respect to didactical courses, however, the potential of online and blended learning seems to be underexplored; little is known about its affordances for teacher education, and for domain specific didactical courses in particular. To investigate this potential, as well as the ways to organize the co-design of such learning units, we carried out a small and short-term research project in which teacher educators in the Netherlands engaged in a co-design process of developing and field-testing open online learning units for mathematics and science didactics. We focused on the features of the designed online learning units, on the organization of the co-design process, and on the experiences with the learning units in teacher education practice. A first conclusion was that it was most fruitful to design building blocks rather than ready-to-use courses, and that students should have play a role in the materials. With respect to the co-design process, intensive meetings of small design teams seemed an efficient approach. The experiences in the field tests revealed that the learning units were inspiring, but needed finalization, and educators needed time to prepare the incorporation in their existing educational practices. In the future, the resulting learning units will be maintained and extended, and are expected to contribute to a community of practice of mathematics and science educators.