The parish church dedicated to S. Pietro and S. Paolo, is the result of different reconstructive attempts and amplifying interventions, realised between the mid nineteenth century and the sixties of the twentieth century.
The archpriest Bertotti commissions the architect Alessandro Antonelli to present a project for the amplification of the parish church which “..with a maximum capacity of 2000 persons it is insufficient to hold a population of 8000 souls”,
The ancient church was almost 56 metres long, with one nave, 16 metres wide and contained 12 chapels. It had a porch with 8 granite columns, whose bases are still conserved in the church’s courtyard.
The church was also attached to the SS. Sacramento oratory and to the ancient cemetery on the south side, while northwards was situated the SS. Rosario oratory, see of the fraternal orders.
Antonelli proposed a first project, of circular shape, then a second one of basilica form. Numerous quarrels followed between the archpriest Bertotti, the mayor Avv. Giacomo Tosi and the local Fabbriceria, because of the high costs the intervention requested. The question was taken to court (Casale Sanate).
For this reason, the mayor had two other projects prepared (which he found more convenient) by the engineer Antonio Busser and the architect Paolo Rivolta.
The edifice was built between 1953 and 1858, according to the project, in a basilica form and at the same time the Fabbriceria opens a new construction site in order to remake the Choir, the presbytery, the belfry and the sacristy, built between 1733 and 1734 according to the last will of Gaspare Gola.
The parish church was blessed in 1858 and consecrated on July 6, 1868, following Antonelli’s project which had foreseen the demolition the S. Fabiano and S. Sebastiano church (see of the fraternal order), in order to make the church’s façade breathe and portion of The San Rocco church (Bishop Seminary), already seat of the Frati Minori Riformati former convent and now seat of the Civic Archeological Ethnographic Museum”C.G. Fanchini”.
The definitive project included the construction of a majestic colonnade, which was never realised, demolishing the entire area around the Town Hall and part of Pozzolo street. The edifice today appears in a Neoclassic style, with a Basilica form and three naves inserted in a semicircular apse.
The façade is proceeded by a pronaos, composed of four granite columns, with composite-style Capitals .
The naves are covered with barrel vaults and above the major altar with a dome vault and apse.
The decorations that cover the dome and the apse were realised in 1898 by the painter Luigi Morgani (from Torino) who collaborated with the decorator Pietro Elia and the firm Minoja from Torino (in charge of the gliding work).
The big stained glass window on the counter-façade represents the Eucharist’s Triumph, created in 1927 by Giovanni Da Re, in remembrance of the Eucharistic Congress held in Oleggio in 1822.
The other stained glasses were commissioned by different institutions and benefactors between 1928 and 1943.
Some altars were built with precious marbles, recycled from the churches that had been demolished during the new parish church construction, such as the altar of Madonna del Rosario and the one in SS. Crocefisso, while others were built new; the altar in SS. Cuore di Gesù e di Maria, the one in Transito di S. Giuseppe, in the Anime del Purgatorio, in San Giovanni were the ancient altar piece is kept, a work attributed to Bernardino Lanino depicting Virgin Mary on a throne, amamong the Saints Giovanni Battista and Evangelista. Finally the altar in San Giovanni Bosco, built in 1946 with the offers of Oleggio’s inhabitants, a way to thank for the end of the Second World War.
On the sides of the nave and the apse one can see the oratories belonging to the once local, fraternal orders , at the time seats of the Religious Art Museum “padre Augusto Mozzetti”.