The monastery was founded on the banks of the Aa in the 7th century by the bishop of Thérouanne, who sent the monks Bertin, Momelin and Ebertram from Omer to proselytize among the pagans in the region.
Saint Audomar (died c. 670), better known as Saint Omer, was a Burgundy-born bishop of Thérouanne, after whom nearby Saint-Omer in northern France was named.
Covered by an armistice in the north, the Flemings raised an army near Cassel, which entered France and attacked Saint-Omer, Terwaan and Tournai.
In the pre-Roman and Roman times the area of the town was located along the border between the Menapii and Morini Belgic tribes of northern Gaul and later the border between the bishoprics of Tournai and Thérouanne.
Saint Silvinus or Silvin (Toulouse, c. 650–Auchy, 15 February 717 or 718) was an evangelist in the area of Thérouanne, which is now in northern France.