In proper english the OA makes a long O sound. Think of all words with oa in them. loan, Groan, Boat, goat etc.... Also the reddish brown cow is a roan. It is not pronounced Ro Un but rone phonetically. Now you may say this is a proper name and the owner of the name has the right to pronounce it any way they choose including mispronouncing it on purpose but until someone can show me someone named Roanoke that pronounces it that way and you are certain the city is named after them it should be pronounce rone oak.