What Is This on Oak Tree?

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On our Oak tree last week, I noticed this green ball that I affectionately named our "Oak Grape" because it looks like a little green grape. I've never seen a "grape" looking thing growing on an Oak tree before. Does anyone know what it may be? It doesn't look like an acorn or any type of cocoon that I have ever seen.
Thank you!
q what is this on oak tree
q what is this on oak tree
  8 answers