Parish Church of St. Saint Stanisław, Bishop and Martyr was built in the years 1905-1907 according to the design of Sylwester Pajzderski, modified by Roger Sławski, in the Neo-Romanesque style (with a clear Rhine provenance). The church was built thanks to the efforts of the parish priest, Fr. Walenty Śmigielski and the parishioners, with the material support of the collector, Prince Ferdynand Radziwiłł. The cornerstone for the new church was laid in 1904. The new church was consecrated on September 30, 1906. The temple was consecrated on October 6, 1906 by Bishop Edward Likowski, later Primate of Poland in 1914-1915. With the establishment of the Kalisz diocese, it was raised to the rank of co-cathedral in the bull of John Paul II "Totus Tuus Poloniae Populus" announced on March 25, 1992.
Because the temple was built in a marshy area, a meter-thick concrete slab was poured under the church, and 200 piles were driven under the tower to a depth of 8 meters. The church is 67 m long and 33 m wide (in the transept), the height of the main nave is 14 m. The tower rises to a height of 55 m. The temple can accommodate 4,000 people. faithful. The facility was built of red semi-clinker brick, while the decorations are made of white sandstone.
The Ostrów co-cathedral is a four-span, three-nave basilica with a transept. The extensive, three-sided presbytery is surrounded by a ring of low, semi-circular apses housing a sacristy, a collator's box and auxiliary rooms. These apses are connected by a corridor and covered with mono-pitched and conical roofs. The asymmetrically situated tower is topped with a pointed four-field roof. In the main nave, transept and side aisles. a groin vault was used, and in the presbytery - a conch vault with lunettes. The walls of the main nave and aisles where the vaults rest are reinforced with buttresses, while in the transept the square towers flanking its corners serve as buttresses. The octagonal shape of the presbytery is topped with a pointed pyramidal roof.
From the west, the Ostrów co-cathedral is decorated with a portico, a rose window and arcaded niches. In the portico, the bas-relief "Annunciation" placed in the tympanum, made by an outstanding artist from Greater Poland, Władysław Marcinkowski, attracts attention. Under the colonnade of the portico you can see a beautiful sculpture "Pilgrim" from 1890, made of sandstone. The sculpture shows the figure of an old man dressed in a long pilgrim's habit, belted with a rope, with a hat on his back, crouching and hugging a rotten cross.
The interior of the church was not initially polychrome, and was colored by beautiful stained glass windows. Currently, the interior is decorated with a polychrome by Henryk Jackowski-Nostitz from 1929.
The spacious octagon-shaped presbytery is decorated with a two-level gallery with 30 sandstone columns supporting a domed vault. Figures of Polish saints and blesseds are placed on the vault of the presbytery. In its central part there is a marble altar, made by Stefan Ballenstedt in 1907, with an alabaster figure of the Virgin Mary and Child, which is adored by two angels carved by Władysław Marcinkowski. In addition to the main altar, there are 6 more altars in the church. Left aisle:
– Our Lady of Perpetual Help (in the apse).
– Saint Antoni.
Right aisle:
– Sacred Heart of Jesus (in the apse).
– Saint Joseph (formerly St. Anna, in a niche).
– Saint Teresa of the Child Jesus (in a niche).
– Our Lady of Częstochowa (in the side chapel)
The interior also includes a marble pulpit on pillars, decorated with a mosaic of St. Peter, holding a key, as well as a marble baptismal font with a forged sheet metal cover. The organ with 33 voices was built ten years before World War II by Józef Stanisławski's company from Poznań.