William Fitz Osbern, was the illegitimate son of Osbern the Seneschal, who became one of the legal guardians of William the Conqueror after the death of his father Robert, Duke of Normandy, in 1035. A number of Norman barons would not accept an illegitimate son as their leader and in 1040 an attempt was made to kill William. The plot failed but they did kill the guardians Osbern the Seneschal, Gilbert of Brionne and Alan of Brittany.
Fitz Osbern became a close friend of William the Conqueror and at the Council of Lillebonne, urged the Norman barons to invade England. According to Norman chroniclers, Fitz Osbern led the right wing of the forces at the Battle of Hastings.
After his coronation in 1066, William the Conqueror claimed that all the land in England now belonged to him. William retained about a fifth of this land for his own use. The rest was distributed to those men who had helped him defeat Harold. William Fitz Osbern was given vast estates, notably in the Welsh Marshes.
After appointing Fitz Osbern and Odo of Bayeux as co-regents, William the Conqueror spent time in Normandy (March to December, 1067). While he was away, disturbances broke out in Kent, Herefordshire, and in the north of the country and Fitz Osbern played a leading role in putting down these rebellions.
To maintain control over his land Fitz Osbern built several castles including those at Chepstow, Clifford and Wigmore. He had particular problems with Edric the Wild in Herefordshire in 1070.
Fitz Osbern was one of William's senior administrators and worked on his behalf in Normandy and Flanders. William Fitz Osbern was killed in a battle at Cassel in February 1071.