The letter "O" is the fifteenth letter in the English alphabet. It is also the fourth vowel letter in the Latin alphabet, used in the modern English alphabet, the alphabets of other western European languages and others worldwide. The uppercase form of the letter "O" is written as "O", while the lowercase form is written as "o". The letter "O" is a vowel and has two sounds, the short sound /o/ and the long sound /oʊ/ .