The building, dedicated to St Peter and Paul, is the reconstruction of an ancient 16th-century church, carried out to designs by the architect Alessandro Antonelli from 1853 to 1858. The imposing Neo-classical façade is preceded by a pronaos with four imposing columns, with capitals ornamented with foliage. The church is built to a basical plan, with a nave and two aisles ending with large semicircular apses. On the interior, late 19th-century ornaments decorate the vaults, the dome and the apse. The painting in the Lombard painter Morazzone (Pier Francesco Mazzucchelli), comes from the ancient church of St Charles. Also noteworthy is the altarpiece ini the south aisle, depicting a Madonna with Child between St Johon the Baptist and St John the Evangelist, painted by Bernardino Lanino.

The church dedicated to St Michael stands in the area of the town cemetery. Documented as far back as 973, the church is mentioned as the parish church of the village in a document from 1347 (from the 16th century the title of parish church of St Peter, located inside the ancient “castrum”). The church has a nave and two aisles ending with semicircular apses, the central being larger and oriented; the presbytery has a nine-step elevation; from the aisles is the entrance to the crypt, having a small nave and aisles, and cross-vaulted ceiling. The gabled façade, west oriented, is decorated with pilasters and blind arches. Really outstanding are the paintings on the interior of the basilica: they are among the few examples of Romanesque painting surviving in northern Italy, after the Counter-Reformation alterations. ON the back of the façade is a Last Judgement in the medieval style; Christ is depicted as the Judge in the middle, flanked by Angels, the Virgin and the Prophets; sadly, the fresco is in very poor condition. Underneath, in the second level, are the busts of the Apostles, while the third level features the Souls of the Faithful. The fresco depicting Knights in the middle of the apse wall is controversial, as the experts do not agree either on the subjects or on the date of the execution. The basilica contains other remarkable frescoes: in the nave is a Resurrection of Christ by Franceschino Cagnola, a painter from Novara. Johannes Maria del Rumo, a painter of Polish origin who lived in Oleggio, is the author of the notable St Michael Archangel on the first pillar to the right.

Not far from town, along the road leading to Arona, stands the oratory dedicated to St Mary, a building with a single nave and a semicircular apse. Documented in 1347 as a chapel of the church of St Peter, it retains of the original structure only the apse and parts of a side wall. Substantial renovations were carried out over the 14th and the 15th centuries. On the interior, the apse wall features the Christ Pantocrator and the Apostles, and in the upper part, the Annunciation. Particularly beautiful is the fresco depicting the Madonna feeding the Child; the frescoes date mainly from the 15th and 26th centuries. Other notable religious buildings from the Romanesque period are scattered in the village and its surroundings. To the west of the town, along the street leading to Mezzomerico, stands the oratory of St Vincent. Romanesque are also the oratory of St Donato, located near the canal Regina Elena. The latter features a gabled façade with two interesting frescoes, depicting a scene of the Plague and an Enthroned Madonna between two Saints surrounded by Souls in Purgatory.

Dedicated to the Birth of the Virgin Mary, this Romanesque oratory stands in the fraction Gaggiolo, a short distance from Oleggio. It is exquisitely decorated: note in particular the frescoes on the south wall, representing the Crucifixion and the Annunciation, both dating from the 15th century. The compositional motifs are typical to the local figurative art and to the Lombard taste.

This building in the Neo-classical style stands on Martiri square. The grand courtyard features porticoes on two sides. On the first floor are some remarkable plasterwork and paintings, including the beautiful ballroom, also named “Sala del Ratto di Deianira” and the room of Aeneas and Dido. The building you see today is the result of a renovation carried out in the late 18th century by the architect Melchioni.

The building of the city museum, a reconstruction from the 1960s, stand in della Chiesa road. Originally a Franciscan convent, the building was altered in the 19th century. The collection was started by cavalier Giacomo Franchini, and has enriched over the years with a number of everyday-life objects, mainly peculiar to farmers and craftsmen of the area. In 1999 the museum has organized an archaeological section.