Its origin and name are due to the monk Audomarus (or Otmarus), who came from Luxeuil around the middle of the 7th century to evangelize the region and later became bishop of Thérouanne. He obtained from Adroald, a local lord, the land at a place called Sithiu and there founded a monastery, to which his monk companions Momelinus, Bertinus, and Ebertram repaired; the first founded the monastery that took his name (St-Momelin), and later became bishop of Noyon; the second founded the monastery that also bore his name (St-Bertin); the monastery was later dedicated to St Otmarus (St-O.), whose relics were kept in a chapel dedicated to the Blessed Virgin; around this monastery the city gradually formed, taking the name St-O. It belonged first to the counts of Flanders, then to the counts of Artois, and later to the dukes of Burgundy. The kings of France occupied it in 1677, and the Treaty of Nijmegen the following year confirmed its possession. The diocese was created after the destruction of Thérouanne (1553) and was suppressed during the Revolution. It was a suffragan of Cambrai; after the Revolution the territory passed to the diocese of Arras, and its bishop retains the title.
The ancient cathedral (Notre-Dame) dates almost entirely from the 13th century. It has vast proportions (100 x 53 m and 22 m in height), three vaulted naves divided by pillars, a transept, and terminates in an apse with an ambulatory. The tomb of St Omer has a 13th-century cenotaph; nearby is the tomb of Eustace de Croÿ, provost of St-O. and later bishop of Arras (d. 1538). After the destruction of Thérouanne, Charles V in 1553 donated to the cathedral the 13th-century group that had adorned the portal of that city, representing the Redeemer between the Blessed Virgin and St John the Baptist. In the transept is the chapel of Notre-Dame-des-Miracles, containing the venerated wooden statue of the Madonna from the 13th century, crowned on 15 July 1875. Also venerated is the tomb of St Ekembaldus (an 8th-century monument). The exterior is dominated by a tall (50 m) square tower completed in 1499. The four portals (13th–14th centuries) are decorated with representations of the Blessed Virgin and the Last Judgement.
Notable are the remains of the church of St-Bertin, adjacent to the abbey (1326–1520); the surviving tower, 58 m high (1431–1520), collapsed on 22 July 1947. The college of St-O. was founded in 1592–93 in the abbey of St-Bertin by Fr R. Person SJ and endowed by Philip II for the education of young men. After the Jesuits were expelled from France in 1773, the students moved to Bruges, then to Liège, and finally to Stonyhurst in England; the college was resumed by the clergy of the College of Douai until 1793, when all were imprisoned; released in 1795, the students returned to England at Old Hall (St Edmund’s College). The city has a rich library: 400,000 volumes, 1,680 manuscripts, and 212 incunabula; notable items include a 13th-century pocket Bible and the so-called St-O. pocket Missal (14th century); the Life of St Omer (8th century); the Gutenberg Bible; and the charter of enfranchisement granted in 1127 by William Clito, count of Flanders, to the city. The fine arts museum is rich in ceramics and paintings; the Henri Dupuis Museum houses collections from the donor and natural history.