The letter "O" in the English alphabet is a what?

The English alphabet comprises vowels and consonants. Vowels are letters that represent sounds that are produced without any restriction of airflow through the mouth or throat. The English vowels are: A, E, I, O, U. Consonants are letters that represent speech sounds that can only be made when the vocal tract is obstructed, either partially or entirely. Consonants require certain positions of the lips, tongue, and cheeks. There are 21 consonants in the English language.

Source: Language Tool