Joe, if there's one thing I'm not it's a latin scholar, and I wouldn't presume to question the Oxford, but if you are after the "original", shouldn't it be "Christus natus est"? I know it doesn't fit the tune so well, but I wonder how much a 16th century Finnish clerical student would change latin grammar in a song for use in church where this would be noticed and commented on. Perhaps the structure of Finnish had some influence? Any ideas?

Pete M