Answer: What you describe are called oak tree galls and they are made by a tiny wasp. The adult lays an egg just below the surface and the tree reacts by growing a gall. The little larvae remains in the gall until it becomes an adult and then flies away. Galls are common on oak trees and do not hurt the tree - in fact their presence gives the tree some character. There's nothing you can do to prevent the wasps from laying their eggs unles you catch them in the act and spray each individual wasp. So, just enjoy your tree and the character it has developed over the years. Enjoy your garden!
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