for an oak-feeding Coptodisca would be C. quercicolella. I don't know what it looks like or if it occurs in California--maybe someone else does?

Both species have been documented in California. However, C. quercicolella has only been recorded from Central CA while C. powellella has been recorded from Orange County(1). Though it must be said that the database has not been updated in some time, and by no means is it complete. I havn't been able to find any plates of C. quercicolella yet, but there is supposedly a description of the species in this article. Unfortunately I cannot access the full article at this time.

Here is Braun's description from that paper--it sounds similar, but maybe not quite the same, what do you think?

"Face pale silvery gray, head strongly tinged with pale golden on vertex and posteriorly; antennae pale gray at base, dark brown toward apex. Thorax and basal half of fore wing silvery gray; wing abruptly darkening at the middle, where the scales are dark reddish bronze and black-tipped. This dark area occupies the whole of the outer dorsal half of the wing, and extends upward in varying width to the middle of the costa. Its outer edge in costal half is concave and sharply defined, and forms the inner black margin of a triangular orange yellow patch bordering the first and brilliant silvery costal spot; very rarely the line of black scales does not reach costa. This yellow patch and the more or less quadrate yellow area immediately posterior to the first silvery costal spot and limited outwardly by the inner black border of the second silvery spot, are the only orange yellow areas in the otherwise dark ground color of the apical half of the wing. A dorsal silvery spot opposite first costal and a few silvery scales on termen below apex. Apical black spot preceded by metallic blue scales; the usual black scales opposite apex and black pencil in the whitish cilia. Hind wings dark gray. Abdomen dark gray, with reddish and greenish iridescence. Expanse, 5 mm."

The description is very similar to Peter's image, though there do appear to be a few minor inconsistancies in my opinion. I do have a tough time interpretting descriptions sometimes, and an image would really make the comparison a lot easier (here is an online reference image I found for C. powellella... cannot find anything else on C. quercicolella though)

In the 1971 paper in which Opler describes C. powellella (J. Lepid. Soc. 25: 194-211), both of the species in question are illustrated in drawings (p. 195). What is indicated is that the orange area of the forewing of C. powellella (Fig. 2) extends apically beyond the opposed silver markings and is interrupted apically by the large black triangle at the apex of the wing, so that the orange color sends one arm to the costal margin of the wing, and another arm to the outer margin. In C. querciella (Fig. 4, which was drawn from one of Braun’s paratypes of that species), the entire posterior half of the forewing is black, starting in the orange area that occurs before the opposing silver marks, and continuing apically for the extent of the wing, so that no orange color touches the outer margin. Given this, the moth in the present photo would be unequivocally diagnosed as C. powellella.

...for the help with ID