What is the author trying to convey with the word 'O' in the following:
He has told you, O man, what is good;
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This is the vocative case:
A vocative expression is an expression of direct address, wherein the identity of the party being spoken to is set forth expressly within a sentence...
Historically, or in poetic or rhetorical speech, the vocative role in English may also be shown by prefacing the noun or noun phrase with the English word "O".
Modern English does not have a formal vocative case, and may use the nominative to fill that role.
The cases are more apparent when looking at some other languages like Latin. A well-known passage from Alice's Adventures in Wonderland where Alice speaks to a mouse reviews them, ending in the vocative:
'O Mouse, do you know the way out of this pool? I am very tired of swimming about here, O Mouse!' (Alice thought this must be the right way of speaking to a mouse: she had never done such a thing before, but she remembered having seen in her brother's Latin Grammar, 'A mouse—of a mouse—to a mouse—a mouse—O mouse!')
Note that the "O" itself can be considered an interjection, much like "Oh" (thanks, @Kit and @Cerberus!); it might be more clear to say it is an interjection accompanying a noun that is in the (unmarked) vocative, "man" in your example.