Where does “о” come from?

о (Mongolian) comes from Russian о, from Old East Slavic о, from Proto-Slavic o(b), from Proto-Balto-Slavic ab, from Proto-Indo-European h₃ebʰi.

Ancestry of “о”, step by step

StepLanguageWordMeaning
1Russianоoh
2Old East Slavicоat, during
3Proto-Slavico(b)around, about; concerning; against, on
4Proto-Balto-Slavicab—
5Proto-Indo-Europeanh₃ebʰi—
Every word from Proto-Indo-European h₃ebʰi →Every word from Proto-Balto-Slavic ab →Every word from Proto-Slavic o(b) →