Note: Do not rely on this information. It is very old.
OO, the 15th letter of the alphabet, and the only one which is not represented in the Egyptian hieroglyphics. It was probably invented at a later date by the Semites. The Semitic name denotes an eye. Before its adoption by the Greeks, who gave it its later value, its sound was like that of h in house. The vowel o is intermediate between a and u, and represents several varieties of sound, according as the cavity of the mouth is more or less contracted. Thus the sounds in not and cord are nearer to that of o than are those in bone and pour, for the formation of which the lips are more fully rounded. Numerous digraphs take the place of the o in bone - of coal, soul, etc. O also denotes the neutral vowel heard in son.