My contact says that the gall doesn't look like an oak gall, but if there was really an association with oak, then the ID might need to be revised.

The scrub oak ID is from Pete's comment on the other photo in this pair. I'm not sufficiently familiar with California oaks to ID it myself. Pete, do you mean Scrub Oak (formal common name for Quercus dumosa or Q. berberidifolia)? If the host plant needs an ID, a view of an ungalled leaf would be helpful in identifying it to species, which could in turn help to identify the gall-maker.

To me this looks like a hardened, frothy secretion--something like a mantid ootheca--applied to a normally formed plant. Pete, do you have any other shots of this object, and/or an estimate of its size?

I explored the possibility of this being a gall and found a similar-looking one:

The gall looks similar, as do the toothed leaves of the plant, which is identified as Coyotebrush (Baccharis pilularis); in turn, that is consistent with Bob Carlson's comments on the first photo of this pair.

I thought the plant was coyotebrush too, but when I did I quick online image search, the leaves I saw didn't match, so I figured I must have misremembered what they look like. This must be a Rhopalomyia californica gall too, though I don't understand why the tissue has that transparent, bubbly appearance.

Yes, I encountered the same problem when most of the online images of Coyote Brush showed smooth, rounded margins...until I did find the toothed ones that I posted above. One additional element of confusion is that the toothed leaves of Coyote Brush look similar to those of Scrub Oak! FYI, here is a link to USDA Forest Service profile of this shrub species that mentions both the variable leaf shape and the midge gall.

Sounds like you've nailed it. I am told that the maker of the gall would presumably be Rhopalomyia californica

Sorry for the delay. It could have been Coyotebrush. It was a small dense clustering of plants around a water fountain, including scrub oak, coyotebrush, blue oak, live oak, and another unidentified scrub. I thought the twig was from one of the scrub oaks, but could have easily have been from a coyotebrush plant that has poked through. Thanks for the help.

Whatever the torymid is, I am told that it has the violaceous coloration of koebelei, but if the gall or non-gall or whatever is really on oak, it might be an undescribed species rather than koebelei.