Question
We have a large oak tree that stands about 45' tall and it is dropping its leaves very slowly beginning from the top.  We think it is probably a disease called "oak wilt."  We are hoping that there might be something we could do save it.

Your advice would be much appreciated.

Answer
With what you discribed it is hard to dignose the problem. Here is a link to a good Oak Wilt source that will show you the signs and controls of oak wilt. With this you should be able to ID it. IF you have problems and you are in the US I would suggest you call your local State Forestry Agency and ask one of their Foresters to come out to your place and help. This is a free service.
http://www.na.fs.fed.us/spfo/pubs/howtos/ht_oakwilt/toc.htm