meaning - What purpose does an '-o' serve?

I have been singing a lot of children’s songs lately, and this afternoon in the car I noticed three songs that add an ‑o to the end of words:

• “He had many a mile to go that night before he reached the town-o” from The Fox (no relation)


• “A rare bog, a rattlin’ bog, the bog down in the valley-o” from The Rattlin’ Bog


• “Day-o, me say day-o, daylight come and me wanna go home” from Day-o

This is not used for engineering a rhyme. I had considered two other possibilities. One is that it is used to extend closed consonant sounds, but this obviously doesn't hold up for valley and day.

My other theory was that it was for fitting meter, but this seems doubtful. You could substitute a comparable word to fit the meter (village instead of town, for instance). Also, valley could easily be stretched to fit. And finally, in the song “Day‑o”, there is a part of the song where the ‑o is dropped, but otherwise the line is the same: “Day, me say day, me say day, me say day‑o...”

Aside from the song lyrics, it seems there are examples of this phenomenon in spoken language that may be related. For example, boyo, bucko, kiddo, daddy‑o, and various nicknames like Rocko, Jacko, and so on.

So where does this ‑o come from, and what purpose does it serve?

Answer

The OED (1st edition—another answer supplies a more recent treatment) regards this as a (usually Scots) variant of older -a, both being common tags on the rhyming words in popular ballads (-o from 1727, -a from 1567). See this and this for examples. Note that the convention is only to record the extra syllable in the first stanza, no doubt to save the printer effort.

At A, inter., 4 OED conjectures that it arose in "the necessary retention of ME. final -e where wanted for measure" (that is, meter). I can adduce no example, nor is it likely that any could be found, since would have occurred in the spoken (or sung) language's evolution—and certainly before printing and a rising middle-class market provided an incentive to record such vulgarities.

Uniting arguments at both A and O, we find it implied that over the course of time this extra syllable came to be regarded as an interjection, Ah! or Oh!, evolved thence into a stock ornament of popular song, and eventually became so identified with the genre that it became a 'signature' of burlesque balladry.

This seems very plausible to me. -o marks the genre—and incidentally provides the balladmaker additional melodic opportunity.

EDIT: I believe this answers your question with respect to your first two quotations. In the third o more likely represents a weary interjection. @tchrist's answer seems to me to address your penultimate paragraph.

I can't compete with @tchrist's Tolkien quotation, but I can provide an instance of 'burlesque', from W.S.Gilbert's Yeomen of the Guard:

I have a song to sing, O!
Sing me your song, O!
It is sung to the moon
by a love-lorn loon
Who fled from the mocking throng, O!
It's the song of a merryman, moping mum,
Whose soul was sad, and whose glance was glum
Who sipped no sup and who craved no crumb
As he sighed for the love of a ladye.

Sullivan's tune's pretty nifty, too.