FITZ-OSBERN, WILLIAM, Earl of Hereford (d. Io71), was an intimate friend of William the Conqueror, and the prin cipal agent in preparing for the invasion of England. He was the son of Osbern the seneschal, one of William's guardians during his minority, who had been murdered by the young duke's unruly barons. Fitz-Osbern founded a monastery at Lire about 104 2. He became seneschal to the duke, and urged on the plans for the invasion of England. At the Conquest he received the earldom of Hereford with the special duty of pushing into Wales. During William's absence in 1067, English affairs were left in the hands of Fitz-Osbern and of Odo, bishop of Bayeux. Fitz-Osbern also acted as William's lieutenant during the rebellions of 1069. Fitz Osbern was one of the feudal lords of the Welsh marches, and built several castles for the defence of the border. In 1070 William sent him to assist Queen Matilda in the government of Normandy. But Richilde, widow of Baldwin VI. of Flanders, having offered to marry him if he would protect her son Arnulf against Robert the Frisian, Fitz-Osbern accepted the proposal and joined Richilde in Flanders. He was killed, fighting against Robert, at Cassel in 1071.