Walter Ophamil or Offamil (fl. 1160–1191), italianised as Gualtiero Offamiglio or Offamilio from Latin Ophamilius, was the archdeacon of Cefalù, dean of Agrigento, and archbishop of Palermo (1168–1191), called "il primo ministro", the first minister of the crown. He was long thought to be an Englishman ("Walter of the Mill") who came to Sicily with Peter of Blois and Stephen du Perche at the direction of Rotrou, Archbishop of Rouen, cousin of Queen Margaret of Navarre, originally as a tutor to the royal children of William I of Sicily and Margaret. 'Of the Mill' is actually an incorrect translation, 'ophamilius' referred to Walter being part of William II's 'familiares regis' or from the Latin protofamiliaris, meaning "first confidante [of the king]". His mother was Bona, a patron of the Abbey of Cluny and a devota et fidelis nostra of the king in 1172. His father is unknown.
Walter's first appearance in the historical record is at court as the Latin tutor of the children of William I in 1160. He rose through the ranks until he was a canon of the Cappella Palatina and a candidate for the vacant archiepiscopal throne of 1168, after the deposition of Stephen du Perche. According to Hugo Falcandus, Walter succeeded "less by election than by violent intrusion." Nevertheless, without the support of the queen regent or of the influential Thomas Becket, his faction bribed Pope Alexander III into confirming his election and he was consecrated in the Cathedral of Palermo on 28 September. He received distinctly double-edged congratulations from Peter of Blois, who refers in a letter to his "humble birth".