The use of this Phoenician letter to get a vowel sound is mainly because of the early Greek alphabets, which adopted the letter "omicron" to stand for the vowel /o/. The letter was adopted with the worth while in the Old Italic alphabets, such as the early Latin alphabet. https://musica-infantil89876.blogdon.net/indicators-on-musica-infantil-you-should-know-53150265