The pictures you took are not of egg sacs.  They are one of the many galls that oaks may get throughout their life.  Galls are made by arthropods and this particular one does not harm the overall health of the plant.  There are no treatments necessary.  The insect stings the plant and releases a hormone the plant interprets as plans for growing a new section of growth.  The insect then sits and feeds as the plant grows around it; consequently providing shelter for the insect.  The damage to the plant is minimal and the insect is generally safe from predation and sometimes from being parasitized.