William FitzOsbern was the son of Osbern the Steward, a nephew of Duchess Gunnor, the wife of Duke Richard I of Normandy.

Osbern married Emma, a daughter of Count Rodulf of Ivry, who was a half-brother of Duke Richard I of Normandy.

After William became King of England, FitzOsbern was made an earl, with major land holdings in Gloucestershire, Herefordshire, Oxfordshire, and the Isle of Wight, and smaller areas under his authority in Berkshire, Dorset, Wiltshire, and Worcestershire.

[1] 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.

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.

During this time FitzOsbern and his followers pushed on westwards into Wales, thus beginning the Norman conquest of the Welsh Kingdom of Gwent.