The parish church of Santa Maria Assunta di Oulx stands on the top of a knoll on the edge of the Borgo Vecchio, a few steps from the 14th century Torre Delfinale. Cited for the first time in a document dating from 1050 to 1061, it underwent numerous changes over time, the most important of which occurred between the end of the eighteenth century and the sixties of the nineteenth century. Of the ancient church only the baptistery, the Rosary chapel and the whole north side have been preserved. The main entrance has decorations made by Jacques Jesse of Embrun and his son François in 1676-77, and there are two plaques indicating the burial place of the members of the Des Ambrois de Névache family, moved to the parish church after the closure to the cult of the church of San Pietro alla Prevostura.
Inside the church, with three naves, houses numerous and significant works of art, migrated from the oldest building or from the church of San Pietro alla Prevostura: the baptismal font covered by a wooden pyramid from the mid-eighteenth century, the canvas depicting the Madonna and Child between Saints Francis and Lorenzo, from 1630, signed by the ducal painter Bernardo Orlando, the seventeenth-century canvas of the altar of San Giacomo, a wooden retable without conclusive gilding in the Rosary chapel, other seventeenth-century paintings, and in the presbytery there is the monumental tripartite retable of the high altar, another prestigious work of painted and gilded wooden carving made by Jacques Jesse of Embrun between 1670 and 1676 and gilded in 1678 by Peter Milander of Antwerp.