What is wrong with my oak tree?

[ArborTech]

This does look like what the others have described. I have seen this on many trees in the past few years. Unfortunately it is usually expensive and involved to treat this. The other key thing to consider to is that the tree will become more stressed and weakend, making it more easily damaged during the winters (and our winters can be tough on healthy trees).

Check out this site:
http://www.na.fs.fed.us/spfo/pubs/fi...st/fidl-ae.htm