Привет, Тициана!
"О" pronounced "o" when it is under stress. In other cases it sounds like "a"
2016年2月13日
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http://www.study-languages-online.com/stress-reduction.html a short article about vowels reduction.
http://www.study-languages-online.com/russian-reading.html - a whole section with some reading excercises, where you can also listen to how the words sound
Google translator may be useful if you want to hear how a word sounds.
Please find another link to the online dictionary. They outline the stressed vowel in red and you can also listen to the word.
Hope that helps.
2016年2月13日
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"Меня зовут Тициана".
O is read like A in unstressed positions.
E.g.:
молоко’ (milk) - the last O is stressed and it is read like O. The first and the second O are not stressed, so that is why we pronounce like [malakO']