Greetings Rich,
Without additional information, my initial response suggests oak wilt. Therefore, this is what you need to do:
(1) If you are genuinely concerned about the health and vigor of your tree, please email some high-quality digital pictures (via email) and tissue samples (foliar and twig) to the University of Illinois Plant Clinic http://plantclinic.cropsci.uiuc.edu/
[follow procedures on website regarding how to submit a tissue sample]
AND...
(2) Contact a local certified arborist, city forester, or your local University of Illinois Extension office horticulturist to examine and evaluate your tree "on-site".
Hope this information helped!
