“Oh, dear!” vs. “Oh dear!”

Question

Which is correct, "Oh, dear!" or "Oh dear!"?

My understanding is that the word oh is an interjection, and should thus be followed by a comma. However, is the second usage the correct one because the two words form a phrase and the word dear is not a vocative here? 

Answer 1

The Google Books results come out quite strongly in favor of omitting the comma, but it depends on how long the “Oh . . .” phrase is. Two-word versions usually do not have it, while longer ones like “Oh for the love of . . .” more often do.

You should inspect these for yourself. I went the first twenty pages of “Oh dear”, and certainly found some with the comma. Not many, but some.

Shorter ones:

• Oh dear
• Oh dear me
• Oh dear God
• Oh God
• Oh yes
• Oh no
• Oh crap
• Oh hell
• Oh shit
• Oh fuck

Longer ones:

• Oh no you don’t
• Oh for goodness’ sake
• Oh my goodness
• Oh for God’s sake
• Oh for fuck’s sake
• Oh my heavens
• Oh for Pete’s sake
• Oh for the love of

As you can see, there are exceptions, but most do not use use the comma after the oh. One old but interesting examples is the line from Shakespeare’s All’s Well That Ends Well, because you can find it variously punctuated.

• O, for the love of laughter, let him fetch his drum;
• Oh, for the love of laughter, let him fetch his drum ;
• Oh ! for the love of laughter, let him fetch his drum ;

The old hymn doesn’t usually use a comma, either:

• O for a thousand tongues to sing my dear Redeemer's praise

I suggest omitting the comma, but I don’t know that there is an actual rule here to cite.

Answer 2

The OED mentions "Oh dear" without a comma. I can't say I've ever seen it with one.