I'm afraid to say that I don't think that there is always a rule, Guest. You'll master the different pronounciations of "o" after spending some time on pronounciation and listening. However, you can get some hint from your dictionary which gives the IPA (International Phonetic Alphabet) phonetic symbols to guide you how to pronounce the word (the British dictionaries, such as Oxford, Longman, ...
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Anonymousbut today I've got a chat with a guy, who claims that in English letter 'o' under the strees will be pronaunced close to 'ah'.When you put it in a context, that guy is right. For instance, compile vs compilation: in the latter, "com" is pronounced like "calm", thanks to the stress. Stress also attracts coda consonants.