Otto (or Oddo) (c. 851 – 30 November 912), called the Illustrious (der Erlauchte) by later authors, was the Duke of Saxony from 880 to his death.
He was father of Henry the Fowler and grandfather of Otto the Great. He also was father-in-law of Zwentibold, Carolingian King of Lotharingia.
He was the younger son of Duke Liudolf of Saxony and his wife Oda of Billung, and succeeded his brother Bruno as duke after the latter's death in battle in 880. His family, named after his father, is called the Liudolfing, after the accession of his grandson Emperor Otto I also the Ottonian dynasty.
By a charter of King Louis the Younger to Gandersheim Abbey dated 26 January 877, the pago Suththuringa (region of South Thuringia) is described as in comitatu Ottonis (in Otto's county). In a charter of 28 January 897, Otto is described as marchio and the pago Eichesfelden (Eichsfeld) is now found to be within his county (march). He was also the lay abbot of Hersfeld Abbey in 908. He was described as magni ducis Oddonis (great duke Otto) by Widukind of Corvey when describing the marriage of his sister, Liutgard, to King Louis.
Otto rarely left Saxony. He was a regional prince and his overlords, Louis the Younger and Emperor Arnulf of Carinthia, with both of whom he was on good terms, rarely interfered in Saxony. In Saxony, Otto was king in practice and he established himself as tributary ruler over the neighbouring Slav tribes, such as the Daleminzi.
According to Widukind of Corvey, Otto was offered the kingship of East Francia after the death of Louis the Child in 911, but did not accept it on account of his advanced age, instead suggesting Conrad of Franconia. The truthfulness of this report is considered doubtful.
Otto's wife was Hathui of Babenberg (Hedwiga, †903), daughter of Henry of Franconia. Otto was and is buried in the church of Gandersheim Abbey. He had two sons, Thankmar and Liudolf, who predeceased him, but his third son Henry succeeded him as duke of Saxony and was later elected king. His daughter Oda married the Carolingian King Zwentibold of Lotharingia.