Otto I, traditionally known as Otto I the Great (German: Otto I. der Große), was German king from 936 and emperor of the Holy Roman Empire from 962 until his death in 973. He was the oldest son of Henry I the Fowler and Matilda.
Otto inherited the Duchy of Saxony and the kingship of the Germans upon his father's death in 936. He continued his father's work of unifying all German tribes into a single kingdom and greatly expanded the king's powers at the expense of the aristocracy. Through strategic marriages and personal appointments, Otto installed members of his family in the kingdom's most important duchies. This reduced the various dukes, who had previously been co-equals with the king, to royal subjects under his authority. Otto transformed the Roman Catholic Church in Germany to strengthen royal authority and subjected its clergy to his personal control.
After putting down a brief civil war among the rebellious duchies, Otto defeated the Magyars at the Battle of Lechfeld in 955, thus ending the Hungarian invasions of Western Europe. The victory against the pagan Magyars earned Otto a reputation as a savior of Christendom and secured his hold over the kingdom. By 961, Otto had conquered the Kingdom of Italy and extended his realm's borders to the north, east, and south. The patronage of Otto and his immediate successors facilitated a so-called "Ottonian Renaissance" of arts and architecture. Following the example of Charlemagne's coronation as "Emperor of the Romans" in 800, Otto was crowned Holy Roman Emperor in 962 by Pope John XII in Rome.
Otto's later years were marked by conflicts with the papacy and struggles to stabilize his rule over Italy. Reigning from Rome, Otto sought to improve relations with the Byzantine Empire, which opposed his claim to emperorship and his realm's further expansion to the south. To resolve this conflict, the Byzantine princess Theophanu married his son Otto II in April 972. Otto finally returned to Germany in August 972 and died at Memleben in May 973. Otto II succeeded him as Holy Roman Emperor.