Russian words have one stressed syllable. The stress may fall on the first, second, third etc. syllable of a word. The  stressed syllable is longer and is articulated more tensely than the unstressed ones. In unstressed syllables the vowels undergo reduction and are shorter, while the vowels [à], [o] and [ý] change their timbre.

O when stressed -> [o]. O when unstressed -> [a]. In an unstressed position [o] is pronounced in the same way as [a] in an identical position.

ÒÀÒÀ

ÒÀÒÀ

ÒÀÒÀÒÀ

 ÝÒÎ [ý-òà]

 ÌÍÎÃÎ [ìíî-ãà]

  ÊÀÐÒÀ

 ÌÀÐÊÀ

 ÃÐÓÏÏÀ

 ÎÍÀ [à-íà]

  ÏÎÃÎÄÀ [ïà-ãî-äà]