Odilo of Mercoeur entered Cluny under Abbot Maieul and succeeded him as fifth abbot in 994, holding the office for fifty-four years — longer than any other Cluniac abbot. Under his rule Cluny grew from a regional reform house into a network of more than a thousand dependent monasteries across Western Europe, the largest religious institution between the patriarchates and the universities.
His most lasting institutional contribution was the establishment, around 998, of November 2 as the universal commemoration of all the faithful departed — All Souls' Day. The practice spread rapidly from the Cluniac network into the whole Latin Church and is observed by Catholics and Anglicans worldwide.
He was a champion of the Peace of God and the Truce of God movements, which used episcopal and monastic authority to limit feudal violence. Personally austere — he lived on bread, salt, and beans for years — he was nonetheless the spiritual director of emperors, popes, and queens, and corresponded as a peer with Henry II and Stephen of Hungary.