Fleming was a prolific letter writer. The Library of the University of North Carolina houses the William Oliver Fleming Papers, which is a very large collection of some 150 items from Fleming (Collection #2292), in the Southern Historical Collection. I don't believe a diary is included, but you may wish to contact the library staff at the following e-mail address: mss@email.unc.edu, or call them at 919/962-1345 and inquire about the collection. Staff was very helpful and they can provide you the online catalog index of items housed in the collection. The actual collection is not on line, but staff will make copies of selected items when you complete a request form. One word of caution, I obtained copies of his letters from 1862-1866. They are generally very difficult to read due to the fading of the letters over time. In addition, several of the same letters are available through the Confederate Reminiscences and Letters Volumes, the Frank Jones book about the history of Decatur County, and even old newspapers. Fleming provided articles to
Southern newspapers during the war, such as the Savannah Republican and others. Some of those articles are found in the old newspaper collection in the University of Georgia Library at Athens.