Otu is one of a small group of Glagolitic letters whose reason for existing is Greek rather than Slavonic. The group also contains the letter for f, which writes a sound Slavonic did not have; the letter answering to Greek theta, used to spell Greek names correctly; and the letter answering to Greek upsilon at the very end of the order. Between them these signs are the clearest evidence in the alphabet of what it was made for.
The mission that produced it was a translation project. Constantine and Methodius were sent from Constantinople to Moravia in the 860s to make the scriptures and the liturgy available in Slavonic, and a script for that purpose has to be able to write Greek proper names, Greek theological vocabulary and the numbers of chapters and psalms as the Greek books gave them. Letters that no Slavic word needs are therefore not a flaw in the design; they are part of the specification.
The same group of letters is the part of the alphabet where a derivation from Greek is least controversial, since these signs correspond to Greek letters by function whatever is decided about their shapes. The atlas records the correspondence for this letter as attested and marks the shape question separately, where it belongs.
The distinction is worth holding on to, because the two questions get run together easily. That a Glagolitic letter answers to a particular Greek letter, occupying its place in the order and doing its work in loanwords, is a matter of observation. Where the Glagolitic letterform came from is a matter of argument, and the same letter can be certain on the first count and unsettled on the second.