Pronunciation of "о" and "по"

Hi ;)

Can you tell me what is the correct pronunciation of the word "о" and "по" in sentences like these ->

Я часто думаю о тебе.
Я лучше говорю по телефону.
Рассказ о моём друге.

Sometimes I can hear you saying A and PA and sometimes O and PO. I'm confused :-o

Yes, right. 'O' in an unstressed position is reduced to 'A' or even to some unclear sound resembling the English shwa sound.

Ok, I understand. But which variant is correct in those sentences and how can I know what pronunciation to use? :)

First you need to check the correct stress. If the syllable with the 'O' vowel is not stressed, pronounce it as 'A' (or shwa). As far as your examples is concerned, both of the afore-mentioned options ('o' and 'по') are prepositions, which means that 'O' should be unstressed. So, it's [pа] for 'ПО' (also [pə], when more reduced) and [a] for 'O' (also [ə], when more reduced).

> Sometimes I can hear you saying A and PA and sometimes O and PO
'a' is just reduction of sound due to fast and fluent speech. If you talk slowly/meditatively this sound shifts a little to 'o', but never gets as sharp and clear as stressed 'o'.
There are two accents in Russia: 'окающий' and 'акающий' ones. 'акающий' is more common.
https://ru.wikipedia.org/wiki/%D0%9E...BD%D1%8C%D0%B5
So, technically you cannot be wrong, you just get accent (imho, countrylike one).

One can get acquainted with оканье here
https://www.youtube.com/watch?v=6mUN64bAkJw
https://alllyr.ru/lyrics/song/111756...sidim-pookaem/