When we describe fitness-for-purpose as the overriding concern for defining the quality of an OER, then this suggests we focus on the learning achieved by the students who use the OER. There are five and only five Domains of Learning. Focusing on achieved learning by students, then the Domains of Learning could be a good Framework as a basis, and onto which to position the various components concerning quality for OER.
According to some reports the quality of an OER should be determined by the subject content material (which is in the Cognitive Domain of Learning), while others have said the OER should be interesting and fun for the student (in the Affective Domain). Built-in self-assessment has also been advocated (in the Metacognitive Domain), accessibility and localisation (in the Environment Domain), and discoverability as well (in the Management Domain) have been suggested.
I am exploring the literature, and welcome your input !
All Best Wishes