I was asked to inspect a hollow in a Willow Oak last month that turned out to be in need of removal. The hollow was too far advanced to recommend leaving it. Here's a shot of the tree.
One thing I should have mentioned, this tree was also the subject of a bad topping (as if there is such a thing as a good topping) some years ago. If you look at the first picture, you can see where it was topped and how the re-growth strated to shoot upward. It was a shorter tree than it should've been judging from the trunk tapper.