Hi Mary,

  That may be possibly a serious condition that affects oak trees. Below is an article on hypoxylon canker.  

https://agrilifeextension.tamu.edu/library/landscaping/hypoxylon-canker-of-oaks/

   It looks from the photo that the limb was possibly damaged in the past and that provided an opportunity for the fungus to invade the tree, if it is hypoxylon canker.  I would recommend you have a certified arborist give you an expert opinion.  If you don't already have a certified arborist, you can find one through the following Trees Are Good link:

https://www.treesaregood.org

    Wishing you and your trees the very best!

Bob Kunkel