OK

When stressed:
О is always [О],
А is always [А].

When unstressed, those two phonems become undistinguishable due to the process of reduction (a phonem is a base phonetic unit of a language).
To put it bluntly, they sound somewhat close to a weak [А]. That's what all Russians know

To be more precise, we have to admit there are two degrees of reduction. No Russian (except lingusts and philologists) is aware about this fact!

1st degree of reduction (reduced, but not too much) always occurs in the syllable which immediately precedes the stressed syllable. О and А in such syllables are pronounced [ɐ] (this vowel is almost [a], but pronounced with the mouth slightly less open).
Please find a detailed description of this vowel (with audio!) here:
Near-open central vowel - Wikipedia, the free encyclopedia
Examples:
самá [sɐ-`ma]
водá [vɐ-`da]
катóк [kɐ-`tok]
акнó [ɐ-`kno]

2nd degree of reduction (strongly reduced) occurs in all other unstressed positions.

In the examples below I underlined the vowels with the first reduction degree, and did not underline the vowels with the second reduction degree. So you can see the unstressed vowels which are not underlined are reduced stronger (i.e. pronounced weaker).

О and А in such syllables are pronounced [ə] (this vowel is usually called schwa).
Please find a detailed description of this vowel (with audio!) here:
Mid-central vowel - Wikipedia, the free encyclopedia



  • Pre-stressed syllables which are farther away from the stressed one (unlike the examples above):

хорошó [xə-rɐ-`ʂo]
карандáш [kə-rɐn-`daʂ]
сапоги́ [sə-pɐ-`gji]
покажи́ [pə-kɐ-`ʐɨ]


  • Any post-stressed syllable:

кóлос [`ko-ləs]
кóмпас [`kom-pəs]
потóмок [pɐ-`to-mək]
закáзано [zɐ-`ka-zə-nə]

Please also note: what I explained is not just a “reading rule”, that is rather a general pronunciation rule, i.e. how russian phonetics works.
And please note: most of the native speakers are not aware about the difference between [a], [ɐ] and [ə], they just think it is [a] sound

There are 2 more nuances about that, but they are not too important for the whole picture, so I will write about them later.