Both Latin o and Cyrillic о originated from Greek letter ο (omicron), which, in turn, was adopted from Phoenician alphabet. According to Wikipedia, the name of Phoenician letter was ʿayin, meaning “eye”, its shape was that of, well, an eye, and it was ultimately derived from the Egyptian hieroglyph ı͗r. (That is, “eye”.)
In many languages, o can constitute a whole word. For example, in Spanish, Italian, Catalan, Tagalog and Tok Pisin o is a conjunction meaning “or”. In Russian and some other Slavic languages, о (or o) is a preposition with a variety of meanings such as “about”, “of”, “on”, “against”, “upon”. In Scots, it is a preposition “of”. In Scottish Gaelic, “from”. In Welsh, both “of” and “from”. In Romanian, a feminine singular indefinite article. In Greek, ο is a masculine singular definite article. In Turkish, a pronoun “he”, “she”, or “it”. In Guaraní, it means “house”.
Oh! Almost forgot: in many, many languages o is an interjection. Meaning “oh” or “O”.
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