We know by heart a lot of words and for some words we have rules. The most important for you is to learn how to read those letters. 1. The basic rule is if the either letter is under-stressed it is been read as it is written (A=A, O=O). 2. If letter O is not under stress - read O as A (and shorter. This is the reduction that makes it easier. Remember that there is only one syllable is been under-stressed in each word in Russian.

Yes, there is such problem that letters "a" and "o" pronounced as "a" and you don't know what to write. Russians check it usually finding words with the same root, but with the stressed "a" or "o". For example: you hear a word [брасо́к] (to throw) And you don't know how to write correctly: брасо́к or бросо́к. There is word with the same root: Бро́сить. [бро́сить] Here we clearly hear "о", consequently root "брос" should be written with "о". бросок - is correct one. To use this rule you need to know many words, and it is good for native and advanced speaker. If you re a beginner, I guess, it is anyway is useful to understand the sense. Also I can remark that not all words can be checked like this. Those words you need to check in dictionary and just remember.

They aren't the same but you read 'o' as 'a' in unstressed syllables. more on vowel reduction here: http://www.livinglanguage.com/community/discussion/434/stress-vowel-reduction-in-russian-4-major-pronunciation-rules/p1