Os is the vowel O
Anglo-Saxon Rune Poem (from ragweedforge.com)
The mouth is the source of all language,
a pillar of wisdom and a comfort to wise men,a blessing and a joy to every knight.
Os (mouth) is the rune of language, communication, eloquence, poetry, song and incantation. I also consider it to be the rune of Odhinn, especially in his role as Galdrfadr (Father of Spells). When it appears reversed, expect troubles with communication, language and other such pursuits. It is rather similar in meaning to that dread thrice annual event that people all seem to fear, Mercury Retrograde. At least, in this case, it is only for day, and it will soon pass. It might also even be easily resolved with aid of other magic that has been done in advance.