outstanding orator? We'll never really know but Letters of Note editor Shaun Usher has pointed out "what might be the first known usage of O.M.G., in a September 1917 missive from British admiral John Arbuthnot "Jacky" Fisher (or Lord Fisher**) to Sir Winston Churchill.". Hat-tip New York Magazine. Click on image to enlarge and read. Certainly the Oxford English Dictionary (OED) suggests it was the first usage.
** Lord Fisher is often considered the second most important figure in British naval history, after Lord Nelson [Wiki]