This monastery dates back to the union of three small hermit communities in the region: one in Oia, grouped around a chapel dedicated to Our Lady; a second one in veneration of Saints Cosmas and Damian, in the parish of Bahíña, and a third one in San Mamede de Loureza. The three came together under the authority of abbot Pedro Laicense and the name of Oia, because this was where the first building for the Benedictine monks was erected.
The first record of the monastery is a donation from King Alfonso VII to the abbot in 1137: half of the churches of Erizana and A Guardia and the ones in Burgueira, Pedornes, Mougás, Loureza and O Rosal. Abbot Pedro Laicense was the head of the monastery until 1157. It incorporated into the Cistercian Order in 1182, and Pelagius II, the first Cistercian abbot, was the head of the community from 1182 to 1186.
The Cistercian reform changed the monastery. Privileges and donations from the monarchy increased and the Cistercian Order grew in the region. This brought about the construction of a new church, of Cistercian style, and an extension to the monastery. A new town was created nearby, now known as Barrio de O Arrabal.
The religious activities were tied to education in some parishes due to the existence of schools for the monks, and to agriculture in others because of horse breeding, which originated popular festivities that are still celebrated today.
In the 15th century (1547), the monastery became a part of the Cistercian Congregation of Castile due to a reform of monasteries, and it lost its independence. The situation had benefits in terms of discipline and observance, but as a result monks in Galician monasteries were sidelined for years.
From the 16th century onwards, a force of volunteer soldiers received some official acknowledgment. They were townsfolk and some monks from the monastic area, who protected the cove and coast near the monastery, which was used by ships to disembark. They defended the area in 1624 and prevented five Turkish vessels from reaching the shore (two of them sank and the others escaped). The King granted a royal title to the church for their efforts.
The monastery, which became a Site of Cultural Interest in 1931, began construction in the mid-12th century thanks to donations from King Alfonso VII. The community followed the Rule of Saint Benedict until it changed to the Cistercian Order. The building has Romanesque, Gothic and Baroque elements due to the different restorations up to the 20th century. The sacristy, high choir, processional cloister and the floor under the monastic cells were built in the 16th century. The old kitchen, pantry and new rooms, as well as an extension to the library and the cellar, are all from the 17th century. Also from this century are the defensive works for the coastline and the fortification of the monastery with walls and a bailey with merlons and embrasures.
In the 18th century, the monastery’s facade was restored and a tower and two aisles were added. In the 19th century, it became a private property due to the ecclesiastical confiscations of Mendizábal and the church was given to the parish. In the 20th century, other spaces such as the infirmary and the chapel were added.
The Portuguese route of the Camino de Santiago (‘Way of Saint James’) starts in Lisboa-Coimbra-Oporto and then it is divided into three paths: the interior or central section, the north section (named after the Portuguese sanctuary of Nossa Señora do Norte) and the coastal section, also known as the ‘monastic path’. For the last one, pilgrims cross the Minho River border and travel from A Guarda to Redondela, where the three Portuguese routes combine. The Portuguese coastal route has existed at least since 1450, and the Monastery of Oia is one of the most important stops along the way.
There is also evidence of the monastery containing an important hospital, which was significant for pilgrims, because at the time there was a large number of them and the monastery served as a place for them to rest and gain their strength back for the rest of the Galician route.