о

Spelling: "о" becomes "об" before the vowels а, э, и, о, у: "об исто́рии" (about history), "об э́том" (about this). Before certain consonant clusters and in fixed combinations it becomes "обо": "обо мне" (about me), "обо всём" (about everything).

Stress: "о", "об" and "обо" are unstressed and are pronounced together with the following word as a single unit. The vowel is reduced to an "a"-like sound: "о тебе́" sounds like "атебе́". Even the two-syllable "обо" carries no stress of its own - both of its vowels stay reduced: "обо мне" sounds like "абамне́". This is unlike two-syllable prepositions such as "пе́ред" or "ме́жду", which do have their own stress.