"O" is pronounced as "A" when it is not under the stress.

Hello, Eefje! The rule is "when "o" is stressed it is read / о /, when it is unstressed it is read / а /". This phonetic phenomenon is called "аканье", and it is one of features of Moscow dialect of Russian language (classical Russian). Examples: молоко́ / малако / мои́ / маи / но / но / (usually in Student books the stress is not marked in monosyllable words, because the syllable is stressed by default) Exceptions: You should pay attention at prepositions which contain "o" letter. They "get connected" to the following nouns and become unstressed. Examples: Коро́бка лежи́т под столо́м. / каропка л'эжыт патсталом / Мы отъе́хали от го́рода на 10 киломе́тров. / мы атйэхал'и атгорада на д'эс'ат' к'илам'этраф / Yevgeniy, italki Russian language Informal Tutor

Yeah it's tricky to recognize the correct pronunciation for me too. In Russian, the letter O is clearly pronounced as O when it is stressed. The remaining Os are as reduced As expressed (that is shorter and less distinct as a stressed A). It is therefore important for me that emphasis realizing in learning the Russian vocabulary. Fixed stress rules do not exist in Russian. I mark the stressed syllable by writing it as a capital letter. Here some examples: 1) In the word "хорошо" (good) is only the last O ​​significantly pronounced as O, because it is emphasized: "хорошO". The unstressed Os are pronounced here than reduced A's. 2) The word "плохо" (bad), however, is stressed on the first syllable: "плOхо". The unstressed O at the end is pronounced as a reduced A. 3) In the word "водка" (vodka), the O significantly pronounced as O, because the stress is on the first syllable: "вОдка". 4) The word "вода" (water) on the other hand is emphasized on the on the last syllable: "водА". The unstressed O is pronounced here as a reduced A. May be this works for you too

In stressed positions о is always [o] In the syllable which comes immediately before the stress о is pronounced as [ɐ] In other unstressed possitions о is pronounced as [ə] Eg.: голосовой /ɡələsɐv'oj/ This is the general rule, but there may be some exceptions in loan words, when o is [o] even in unstressed possition.

The word когдА for instance.