Moved from Comb-clawed Beetles.
See more detailed comment with other image.

It's a very pretty tenebrionid.

due its 554 tarsal formula.
It might belong to the subfamily Alleculidae.

This critter is sometimes called a false Bombadier beetle because of the odor it gives off. I believe it is from the genus Chlaenius...maybe species cumatillis....at least 13 species of chlaenius are found in California. They feed on an assortment of plant and animal matter. They were probably raiding the gall looking for food or a hiding place. Isn't it ammazing the the number of creatures that can be found on Oak trees...they are a whole world unto themselves.