The church of Sant Andreu is linked to the history of the castle and the primitive town of Oliana. It was consecrated by Bishop Eribau of Urgell around 1040. In 1978 it was restored and reopened for worship. The building has a single nave, topped to the east by a semicircular apse that opens onto the nave through a short presbytery section.

The nave is covered with a barrel vault reinforced by three toral arches.

There are three windows in the church, one double-hung window in the apse, one in the south façade and one single-hung window in the west wall. Two access doors open on the north wall, on the opposite side of the castle that was next to the church.

The bell tower is square in plan. It consists of a high plinth and two storeys, the last of which has been restored. In the apse, which is very high due to the unevenness of the terrain, is the decoration formed by a frieze of blind arches between lesenes. The church of Sant Andreu is considered to date from the 11th century and was listed as a Cultural Asset of National Interest in 1949.