Ah I've just discovered that it should be it should be "Caller o'u", not "Caller ou", and that o'u is an abbreviation for "oysters", the creel lassies in the song being a reference to the girls who carried oysters for sale round the Edinburgh tenements in their creels. See here.

Furthermore, it seems that google's intelligent search algorithm let me down for once - as soon as I googled the correct spelling of the song (i.e. including the apostrophe), the song's origins immediately became apparent, and in fact were covered in an earlier mudcat thread ( here). It seems it was written by a John Gray sometime early in the 19th century. Also, the lyrics posted in that other mudcat thread are more accurate than the ones I posted.