It certainly could be oak wilt by the description of falling leaves.  Even if you got a sample to the Diagnostic Clinic, if it is oak wilt, there is nothing you can do for the tree.

If you cannot get a sample, then just wait and see if the tree dies.  You can't treat it at this point.  Oak wilt can be prevented with fungicide injections into the trunk but it won't work if the tree already has oak wilt.  Sorry, this is the sad reality.

Thank you Gretchen, my main concern is spreading to other high value oaks on the property. It's not in an area that I could use vibratory plowing to separate root graphs due to underground utilities and above ground structures. Should I wait and see if it dies....or cut it down if suspect. Not sure how fast I need to respond to prevent it's spread.

Also, I think it's too late in the season to treat the other (assumed) healthy trees, correct? They need to be injected in the spring to get the fungicide up into the tree as I understand it.

See this link on sample submission for oak wilt:
http://msue.anr.msu.edu/news/oak_wilt_diagnosing_and_preventing
You may want to call Diagnostic Services<personal data hidden> to see if alternate sampling is a possibility.