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.
In 870 he had married Richilde, who was descended from a noble family of Lorraine, but none of the children whom he had by her played a part of any importance.
Moved to Mons at the beginning of that century, and his only child - a daughter - Richilde, married Baldwin VI.
In 1070 led to civil war in Flanders, the claim of Robert to the guardianship of his nephew Arnulf being disputed by Richilde, the widow of Baldwin.
The issue was decided by the decisive victory of Robert at Cassel (February 1071) when Arnulf was killed and Richilde taken prisoner (see Flanders).