Os is the vowel O (oh)
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 means mouth. It is highly favored in the Anglo-saxon poem, as the mouth gives the ability of speech and language, which allows for the sharing of knowledge, information and wisdom, which is a benefit to everyone.
Esoterically, Os is very much the same. It signifies that speech and language is going to play an important role, and may even be the key to success in your day. This also connects to words and writing as well, because they are just symbols to express what people are saying or have said. Os can also indicate divine utterance, like prophecy or being inspired to speak by spiritual forces, like glossolalia (ie speaking in tongues). In that sense it can also indicate possession, of a very limited nature, through which incorporeal entities might speak through someone. Of course, this power also connects song and poetry, things which were highly valued in the Northern Tradition.