Question: Why is О often pronounced as А?
Answer: Because O is unstressed. The rule is: pronounce О as О if it's stressed, pronounce О as А if it's unstressed.
Question: How do I know if it's unstressed?
Answer: It's not possible to understand if it's stressed or unstressed just by looking at the word.
Question: So what do I do if I want to know when to say А instead of О?
Answer: Memorize how to pronounce the word when you open a dictionary.
Question: Ok, let's say I see МОЛОКО in a sentence. How do I understand how to read it?
Answer: Open a dictionary, find МОЛОКО. You will see a little tick above the stressed O, like this: МОЛОКÓ. It means, that Ó is pronounced as O, and the other ones are pronounced as А. You will also see a full transcription (pronunciation) next to the word.
Question: So does it means that I will have to open a dictionary every time I see a new word in order to say in correctly?
Answer: Yes. Opening a dictionary is like holding a guitar when you are learning to play. If you don't take guitar in your hands, you will not learn how to play it.