Onu writes the vowel o and takes the numeral eighty. Its name is a demonstrative pronoun, that one, and it is the last of the letter names in the first half of the alphabet whose meaning is beyond dispute.
The vowel is a back mid vowel, and Old Church Slavonic uses it constantly, in prepositions, in prefixes and in a large share of its noun endings. It is also one of two signs in the alphabet for a rounded back vowel, the other standing much further down the order, and as with the two i letters the pair does not record two sounds in Slavonic. The second sign answers to a Greek letter that had come to be read as o, and it was carried across with the order.
The numeral eighty follows from counting the Glagolitic letters in sequence, this being the seventeenth sign.
Because the letter is so common it is one of the first shapes a reader of the script learns, and it is also one of the shapes on which the argument about the origin of the Glagolitic forms has often focused, since a rounded script built largely from circles and loops invites the question of whether the circle carries a meaning. That reading is one position among several, and the atlas records it as such.
The letter also does duty in the numeral system more often than most, because eighty falls within the range of figures a manuscript uses constantly for chapters, for psalms and for the gatherings of a codex. A reader of an early Slavonic book therefore meets this sign as a number on many pages, set off by points and covered by a stroke, and has to be able to switch between the two readings without hesitating.