Construction of the Santa María de Oia Monastery, the municipality's main monument, began in the mid-12th century, during the reign of Galician King Alfonso VII. Within its walls gathered monks who previously lived in various parts of the region. In 1185 it joined the Cistercian Order. It has Romanesque, Gothic and Baroque elements, resulting from the different reforms and modifications carried out until the late 18th century. The church has three naves laid out in a Latin Cross design. The choir is 17th century, and the facade is in late-18th century Baroque style.
Thanks to its strategic location (right beside the sea), Santa María de Oia played an important role in defending the coast. In 1624 the monks managed to thwart an attack by the Turkish fleet, for which Philip IV granted the monastery the right to use "Royal" in its name. Following the 1835 Disentailment Act it passed into private hands, although the church serves as parish church.