Galls are abnormal growths caused by a "gall causer" which may be a wasp, mite, wooly aphid, sawfly, fungus, bacteria.
The English Oak has at least 40 gall causers causing around 50 different galls.
Generally the gall causer is a parasite and each gall causer may have parasitoids, and galls may have inquilines (lodgers)... and these may have even small parasites. One Oak Marble Gall contained 24 different insects. So if you see an exit hole it doesn't mean the gall is empty!