In addition to Curt's answer: о is also used with the accusative case when talking about an object coming in physical contact with another object: Он споткнулся о камень и ударился головой о стену -- He tripped over the stone and hit his head against the wall Корабль разбился о скалы -- The ship broke up on the rocks Она вытерла руки о полотенце -- She wiped her hands on a towel (=Она вытерла руки полотенцем)

О is like 'about' in English for contexts like these: Мы думали о Саше. - We were thinking about Sasha. О чём вы говорили? - What were you talking about? As you know, it's used with the prepositional case (locative) when it has this meaning. Its pronunciation goes by the normal rules: prepositions are usually not stressed, and unstressed о sounds like [а] - so "о чём?" sounds like [a chom].

Ok thank you very much

'o' is always pronounced like [a] or [ə] when unstressed.

Ok so the pronunciation is always like a [a] and no like a [o] all right ?