Coptic Small Letter Oou

It is Coptic omega, a vowel letter for an o sound.

Coptic is the later form of Egyptian, written with a Greek-based alphabet. Old Coptic writing was in use by the first century CE at latest; standardized Coptic emerged in the third century CE. Unlike older Egyptian scripts, which usually left vowels unmarked, Coptic spelling wrote them out. That helped ritual and astrological texts preserve pronunciation; Christian communities later used Coptic for scripture.

In Sahidic, a Coptic dialect, ⲱ is usually transliterated ō; the line over o marks a long vowel. Bohairic, another Coptic dialect, distinguishes ⲱ from ⲟⲩ in its teaching materials, so dialect affects how the vowel is read. Coptic fonts may also give the same letter different Sahidic and Bohairic styles.