What is the meaning of -오?

Post-vowel and post- (l) allomorphic form of (-so, semi-formal style sentence-final suffix).

Yang-vowel form of (-wu, adverb-deriving suffix)

Lenited allomorphic form of 고〮 (-kwó, verbal connective suffix)

Lenited allomorphic form of 고〮 (-kwo, non-polar interrogative marker)

Source: wiktionary.org