Förstemann, Altdeutsches Namenbuch (1856), s.v. "Aud" (161–180). Förstemann calls the element "excessively frequent" (Ein unendlich häufiger wortstamm). Spelling variants listed for the short form are: Audo, Auto, Oudo, Outo, Outho, Aoto, Aotto, Oato, Odo, Odda, Oddo, Oto, Otto (8th century), Otho, Ottho, Odto, Hodo, Hoto, Hotto, Hottho, Ootto, Ocdo, and Octto. The surviving signatures of the Ottoian kings mostly read Otto, sometimes Odo or Oto. Listed as variants surviving into Modern High German are: Hoth, Hotho, Oette, Ott, Otte, Otto, and Otho. The similarity of the Roman cognomen Otho is entirely coincidental. The spelling Otto is first recorded s.a. 744 in the charters of the Diocese of Constance (ed. Neugart, codex diplomaticus Alamanniae, 1791) and becomes increasingly common in the High Medieval period.
