Otto IPronunciation: /ˈɒtəʊ/Definition of Otto I (912–73), king of the Germans 936–73, Holy Roman emperor 962–73; known as Otto the Great. As king of the Germans he carried out a policy of eastward expansion and as Holy Roman emperor he established a presence in Italy to rival that of the papacy. Otto I in other Oxford dictionaries Definition of Otto I in the US English dictionary