Otu is the second Glagolitic sign for a rounded back vowel and it takes the numeral seven hundred. It answers to Greek omega, and like the second i letter earlier in the alphabet it exists because the order was built on Greek rather than because Slavonic needed two signs for one sound.
The letter is used chiefly in Greek words and names and in certain fixed spellings, and its distribution in a manuscript is a matter of scribal convention. A text that uses it freely and a text that restricts it to loanwords are following different norms, and the difference is one of the small habits by which manuscript families are told apart.
Its name is among the opaque ones, and the reference literature records it without an etymology.
The numeral seven hundred is where the two Slavonic numeral systems part for the last time. Glagolitic counts to twenty five and assigns seven hundred. Cyrillic assigns eight hundred, the value of Greek omega, to the same letter, and gives seven hundred to a Greek letter that Glagolitic does not have at all. From here Glagolitic numbers nothing further, while Cyrillic has three more numbered letters to go.
That is the end of the counted series in this script, and it falls where it does for a reason worth stating. Everything after this point in the Glagolitic order is a Slavic sign: the affricates, the reduced vowels, the nasal vowels and the letters for the glide sequences. Greek had none of them, and Glagolitic, having counted straight through the alphabet to reach seven hundred, simply stops rather than continuing into the thousands. The alphabet therefore has a numbered half and an unnumbered half, and the join between them is where the Greek order runs out and the Slavonic material begins.