The letter "o" is the 15th letter of the English alphabet. It is a vowel and can represent different sounds, such as the long sound in "go" and the short sound in "hot." In written language, "o" is often used in various words and plays a crucial role in forming syllables. In mathematics, "o" can also represent a circle or the concept of zero. In music, it is used in the solfège system as "do," which is the first note of the scale. The letter "o" appears in many languages and is essential for communication.