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).