The jaw of Oligokyphus is odd. For all of the Mammalian characteristics that are present in the body and impression fossils (or carbon films) of Oligokyphus, the jaw is still reptilian enough to have called for a reclassifying of this animal as a reptile. The quadrate, a bone in the area where the lower jaw (in reptiles a complex set of bones and in mammals the singular mandible or dentary) meets with the skull, abuts the squamosal, a bone of the post orbital skull. In most Therapsids quadrate and articular are seen as ear bones already; mammals have 3 ear bones consisting of what once were the quadrate, articular/prearticular, and angular bones in reptilian jaws. The fact that Oligokyphus still possesses a distinct quadrate makes it a very basal (at best) mammal and rather likely that it is actually still more reptilian than mammalian in its composition. Therapsids, Cynodonts, and Tritylodonts (the family of Oligokyphus which is named so due to its tricuspid teeth) are a tricky set of animals. These animals are jigsaw puzzles of transitional skeletal elements linking reptilian and mammalian heritages. Oligokyphus is one of the major players in that jigsaw puzzle of anatomy.
The overall anatomy of Oligokyphus is, as has been noted, rather mammalian with the notable exception of the skull morphology. The high sagittal crest on the skull is apparent in the skull image, but it is not truly appreciated until seen in respect to the entire body. The sagittal crest allows for increased muscle mass and attachment in the skull and typically adds to the closing of the jaw, meaning that the bite force of Oligokyphus was probably fairly significant for a 20 inch (50cm) long animal. All of that bite force, however, was dedicated to the grinding of plant matter; Tritylodonts, and Oligokyphus in particular, were strictly herbivorous. The rest of the weasel-like body would have been dedicated to digesting the vegetable matter that was ingested.