This is a monumental instrument – the largest and most important neoclassical-style organ in the Spanish State. Built by the OESA, a company based in Azpeitia, also in the Basque province of Gipuzkoa, thanks to a legacy left by Fermín de Lasala, the Duke of Mandas.
This organ is accompanied by a smaller one located in the presbytery, each being connected to the other. It operates with an electric system. In 2007 it was re-inaugurated after considerable restoration undertaken by Bernal-Korta of Azpeitia, respecting its characteristic features such as basic acoustics. The old console, nevertheless, has been replaced by a new one with the incorporation of an electronic adjustable combination system.
It has five manual keyboards, each with a compass of 61 notes (C-C) and a pedalboard of 30 (C-F), with a total of 125 stops.