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Thanks for the clarification :)

послезавтра - Wiktionary
радио - Wiktionary
?

Both have sound clips which have a definite «о» sound in. As for «хаос», its Wiktionary entry doesn't have a sound clip, but I asked a Russian...

I believe certain words have unstressed O that is always pronounced as O.

e.g.
ра́дио - radio
ха́ос - chaos
послеза́втра - the day after tomorrow

but this is most certainly the exception...