On a recent Canada Day, someone on LinkedIn referred to O’Canada, which is clearly wrong. (O’ is confined to Irish surnames, where it is the anglicised version of the Gaelic Ó or Ua, meaning ‘descendant of’; M(a)c [or M’], as for the Scots, means ‘son of’).
But is it O Canada, Oh! Canada or what?
O, not followed by punctuation and closely linked in sense to what follows, is what’s called a vocative – and is correct in things like national anthems or hymns.
Oh is usually followed by punctuation and is more like a stand-alone exclamation tacked on to the rest: Oh, hell! Oh! what a scoundrel! Oh, no you don’t!
Usage of O and Oh has not always been consistent, however (think back to the revue Oh! Calcutta! if you’re old enough).
But O’Canada has never been OK.