The HIERONIMO Dictionary

o sir

Definition:
Discoursive phrase to cover a silence. (Crystal and Crystal 2008)

Category:
Discourse marker - Interlocutor's response

Examples:

1
O sir.

Who speaks: Gasparo.
To whom: Lodovico.
Observation:
it can be also considered as a negative exclamation with nuances of regret or lamentation (= O me!)

Translations: