Found this grown on one tree
These are galls from one of a number of oak gall wasps. The likely species in this case is California gallfly (Andricus californicus), which is known for the large apple-like galls that it stimulates the oaks to grow. Gall wasps or their larvae produce chemicals (phytotoxins) that stimulate the host plant to grow the gall structures. Galls provide a relatively safe and nutritious place for the young wasps to develop. See the links below for more information.