Coptic was written in several dialects, and the differences between them fall most heavily on the vowels, which is exactly where the Greek alphabet made Egyptian visible for the first time. Sahidic, associated with the Nile valley of Upper Egypt, became the dominant literary form through the first Christian millennium and is the dialect in which most of the classic Coptic literature survives, including the Nag Hammadi codices and the great monastic corpora.
Bohairic, of the Delta, later became the language of the Coptic Orthodox Church and remains its liturgical language. Alongside these are Akhmimic, Lycopolitan, Fayyumic and Middle Egyptian or Oxyrhynchite, each attested by a smaller body of text and each with its own vowel habits. Ou is one of the letters whose distribution separates them. The relationships between the dialects, and how far each reflects a genuinely regional spoken form as opposed to a scribal tradition, are matters on which specialists differ, and the surviving evidence is uneven in ways that make the question hard to settle: some dialects are known from a handful of manuscripts, and the geography of a find is not the geography of a text. What is not in doubt is that the differences are real, systematic, and concentrated in the vowels.