Saint-Omer is a commune in France.It is a commune and sub-prefecture of the Pas-de-Calais department 68 km (42 mi) west-northwest of Lille on the railway to Calais. The town is named after Saint Audomar, who brought Christianity to the area.The canalised portion of the river Aa begins at Saint-Omer, reaching the North Sea at Gravelines in northern France. Below its walls, the Aa connects with the Neufossé Canal, which ends at the Lys River.