The eighteen Armanen runes are Guido von List's, published in 1908. Reading one of them means separating the attested character underneath from the layer he put on top.
The character behind it
Os corresponds to the Younger Futhark character whose Old Norse name was oss or ass, meaning a god of the Aesir, and whose Old English counterpart carried the name os. The Old English word can mean either a god or a mouth, and the rune poem plays on both. The character's sound value shifted across the Norse and English traditions, which is why its transliteration varies between A and O depending on the corpus.
What von List did with it
He fixed the value at O, retitled the character Os, and assigned it a meaning within his own system. The genuine ambiguity in the historical record is not what he was recording; he was building a row of eighteen and needed a vowel in this position.
The claim and the reception
Von List presented the eighteen as the recovered alphabet of an ancient Germanic priesthood, with the count set by the eighteen rune-charms of the Havamal. Historians of runology treat the row as an early twentieth-century construction and do not accept the recovery claim.
The Armanen row was subsequently taken up in volkisch circles and by National Socialist organisations during the 1930s and 1940s. That is documented, and the atlas states it rather than leaving it out.