By stress (first syllable of о́зеро, second syllable of озёра) and pronounciation of е/ё: о́зеро /'ozʲɪrə/, озёра /ɐ'zʲorə/ (in Cyrillic transcription, [о́з’ьръ] and [ʌз’о́ръ] respectively).
This is the standard pronunciation.
О is only distinguishable from А when it's stressed. Here, the first О is stressed: о́зеро /'ozʲɪrə/ so it's distinguishable.
When О is not stressed, it is pronounced as something in-between О and А, but closer to А (we mark this sound /ə/ or /ɐ/ in IPA transcription, [ъ] or [ʌ] in Cyrillic transcription).