Tempus is a third declension noun (tempus, temporis) and forms its vocative identically to the nominative - hence o tempus.
I almost went with tempus . . . but couldn't remember if it was a 3rd declension or not.
I need to brush up on my Latin grammar . . . and my Greek grammar . . . and my Gaelic grammar . . . and while we're at it I need to review the English too . . . < g >