The Parish Church of Our Lady of the O is the main parish church of Sanlúcar de Barrameda. It was built around 1360, sponsored by Isabel de la Cerda y Guzmán, 1st Countess of Medinaceli and granddaughter of the first Lord of Sanlúcar, Guzmán el Bueno, making use of one of the towers of the old Alcázar which served as its bell tower.
The architectural style follows the typology of Mudéjar parish churches in the Kingdom of Seville , with a rectangular floor plan and a basilica-style interior, consisting of three naves and an octagonal polygonal apse at the head. The central nave is separated from the side aisles by arcades.
The Mudéjar sandstone portal at the foot of the nave is particularly noteworthy. Above it are the coats of arms of the Guzmán and de la Cerda families, supported by rampant lions. On the right side of the church is a second, simpler stone portal with a pointed arch, crowned by a cornice also decorated with grotesque masks.Inside we find an impressive Mudejar coffered ceiling (16th century) and beautiful religious images, some of them from the 17th century.