Re: no sub floor
If the existing flooring isn't split or cracked then overlay with 3/4 t&g would work.
It's likely the old boards probably have become loose over the years so the nails would need to be reset and you might consider putting in a ring shank nail or screw into each plank to insure they're fastened securely.... before laying the new sheathing.
If there will be areas of the old floor will be in need of replacing .... like bathrooms / kitchens ... there will be the need for matching theheight of the replacement floor with the rest.
Just a thought.
"" an ounce of perception -- a pound of obscure "