This parish is where St Omer was born in the 7th century and during the same period may have housed the monastery of Potentinus which followed the austere Irish rule of St Columba, but disappeared during the Viking destructions. Perhaps it was to mark the memory of this Merovingian foundation that the Benedictine abbey of Lessay made the church of St Hélène in Orval a priory church in 1115. Perhaps a Romanesque crypt was built for the same reason, to affirm, as at Saint-Marcouf, on the east coast of the Cotentin region, the continuity of monasticism after the interruption of the Scandinavian invasions.
The nave, which is long and narrow, covered by a wooden ceiling, has herringbone pattern masonry on the outside walls and its roof is supported on a corbel table. The original openings have been subsequently enlarged. The crossing, which has a square ground plan, has four solid arches, with double roll mouldings, which support the lantern tower. The first stage of the lantern tower is still Romanesque, articulated by flat buttresses and pierced on each face by two small round-headed openings supported on a small central column. The second stage belongs to the 13th century gothic, like the eight ribs of the vault over the crossing. The octagonal stair turret is later still (15th century).
The choir and chapels are also gothic (and were reconstructed in the 15th century).
The crypt, which was formerly entered from the transepts through two angled corridors, seems to have been intended to house relics. Only the first compartment, with rib vaults supported by a large central pillar of quadrangular cross-section, belongs to the Romanesque period. The second was added during the reconstruction of the choir.