1) the interjection, O!
Part of Speech: interjection
As a sign of the vocative O; as a note of exclamation. oh: - O.
Perseus Digital Library
University of Chicago's Logion lexicon
Thayer's dictionary plus other information.
From this site's dictionary (in Italian)
Click on the first column to search for that word as a form of the root ὦ; click on the second column to search for that grammatical form of the root ὦ; click on the third column to search for that word and grammatical form; click on the total to search for the root ὦ.