As far as I know, DO and MD residency accreditation councils merged and there won't be separate MD or DO residencies much longer.
Regardless, a DO doing an ACGME residency does not become an MD--they have a DO. Just as an MD would not become DO if they did and osteopathic residency (if that's even allowed).
I don't see why it matters. As an attending, whether someone is a DO or MD makes exactly zero difference to me.