The OER portal twillo provides information on its website about the underlying understanding of quality and lists seven quality criteria that generally apply to all OER.
Further information on the understanding of quality and the seven quality criteria can be found here.
The quality check, which twillo provides in the form of a self-check, is based on these seven quality criteria. With this, OER creators can check the quality of their educational material during conception and development (or afterwards).
Further information on the quality check can be found here.