William FitzOsbern was the son of Osbern the Steward, a nephew of Duchess Gunnor, the wife of Duke Richard I of Normandy. Osbern was the steward of his cousin Duke Robert I of Normandy. When Robert left the Duchy to his young son William, Osbern became one of Duke William's guardians. Osbern married Emma, a daughter of Count Rodolf of Ivry, who was a half-brother of Duke Richard I of Normandy. Through her he inherited a large property in central Normandy, including the honours of Pacy and Breteuil.
As William the Bastard became King of England, FitzOsbern was given charge of the Isle of Wight, and then before 22 February 1067 he was created Earl of Hereford as well as Earl of Gloucester, Earl of Worcester and Earl of Oxfordshire. That western part of England was not yet fully under Norman control; the understanding must have been that FitzOsbern was to take charge of the conquest of these regions when he was able. In the summer of 1067 King William returned to Normandy and left his half-brother Bishop Odo of Bayeux and FitzOsbern in charge of England during his absence. The King was back in England in 1068 and FitzOsbern accompanied him in the subjugation of south-west England. He attended the King's Whitsun court in May 1068, and then visited Normandy, where he fell ill for some months. In February or March 1069 FitzOsbern was asked by William to oversee the peace in York, where Gilbert de Ghent was made castellan of the new castle, but FitzOsbern returned south in time to attend the King's Easter court in April 1069 before returning to York. Eadric the Wild launched a campaign of Anglo-Saxon resistance in the West Midlands, with the assistance of a number of Welsh princes. In 1069 the revolt was crushed, and it is likely FitzOsbern played a major part in this, although the details are not certain. During this time FitzOsbern and his followers pushed on westwards into Wales, thus beginning the Norman conquest of the Welsh Kingdom of Gwent.
In 1070 trouble arose in Flanders, where King William's brother-in-law Count Baldwin VI of Flanders had died, leaving his county and his young sons in the hands of his widow Richilde, Countess of Mons and Hainaut. Her control of Flanders was challenged by the brother of her late husband, Robert the Frisian. Looking for help, she offered herself in marriage to FitzOsbern. He could not resist the chance to become also Count of this rich principality close to Normandy, and hurried there with his army, where he was defeated by the Count of Flanders and killed in the Battle of Cassel on 22 February 1071.
*William of Breteuil, who succeeded his father in Normandy. He was held captive and tortured by Ascelin Gouel de Perceval 'Lupus', Sire d'Yvry, until he finally granted his daughter Isabella de Breteuil in marriage to him.
*Roger de Breteuil, 2nd Earl of Hereford, who succeeded his father in England and Wales;
Secondly, it must be assumed, in 1070 he married Richilde, Countess of Mons and Hainaut shortly before the Battle of Cassel in 1071 by issuing a male descendent who was taken to the Burgundy region of Savoy.
