I assure you that Orange Blossom Special was definitely originally written/played on harp. With time, and some o' dat good ole cheurrah bounce it evolved into a song primarily considered as a fiddle tune. Good, or great harp players can kill on this one! Mike here is correct about how to play it cross harp style, and the switching harps is a great stage gimmick. For harmonica: The D is dominant, A is the key to get her chugging up over Hogback Mountain with. Then let her rip down Saluda Grade just as wide open as you can play the D. My favorite way to play it is trading off at least one lead with a good fiddler, and a dual lead with the fiddler.