We were on our morning walk yesterday, and I happened to look up into one of the California coast live oak trees by the sidewalk, and saw LOADS of oak galls in it!
I think they are marble galls, at least that's what they look like to my untrained eye. There's another kind called oak apples, but they seem to be whiter than these that we found.
This field guide put out by US Department of Agriculture has a chapter on gall wasps on
p. 28. They describe them as oak apples.
Looks like the wasps that developed in this one already left via the exit holes.
It's nice to know that if we ever need a source for making iron gall ink, we've got a great local crop!
Happy gall spotting!