Once classified with the juniper titmouse as a subspecies of the aptly named plain titmouse, the oak titmouse is easily separable from other timice species except the juniper. Its lack of field marks is, ironically, an important field mark. Polytypic. Length 5".
The bird is a drab brownish or grayish brown color overall, but is paler below. It has a short crest.
Four subspecies, one restricted to Baja California; variation is weak and clinal. The northernmost subspecies, inornatus, from Oregon to south-central California, has medium brownish gray or olive-brown upperparts, pale gray underparts with pale brown tinge on flanks, and a short bill. The affabilis found in southwestern California is notably larger, has darker gray-brown or olive-brown upperparts, flanks washed in dusky