Located halfway between Novara and the Lake Maggiore, Oleggio rises in an area inhabited since the ancient times by Liguri and Celtic tribes. It was conquered by the Romans and became part of the Gallia Transpadana. After the fall of the Roman Empire, it passed under the control of the Lombards, first, and of Galeazzo Visconti in 1301, later. Subsequently it became an autonomous municipality, under the influence of the Sforza family. After the French and Spanish invasion, in 1713 the territory was conquered by the Asburgos until the advent of Carlo Emanuele III of the Savoia family.
The town's name probably derives from Olesin, meaning "hill on the Ticino river" (ol = hill, esin = the river).

Not to miss:
- the Bell tower, also called Tower of the Bagliotti, which is the symbol of the town. It is located in Oleggio's main square and is entitled to the Martyrs of Liberty.
- the remains of the ancient city walls and of the Guandra moat, which are still visible in "Motto dei cani" area, Porta Pozzolo and Porta di Costa dei Mazzeri.
- the Parish church of San Michele, built in the city graveyard, is a splendid Romanesque style monument, already mentioned in a document of 973. It features three naves and semicircular apses and preserves inside valuable XI-XII century frescoes, one of the few examples of Romanesque style paintings in the Northern Italy.
- the Church of Saints Pietro and Paolo was projected in Neoclassical style by Alessandro Antonelli and built between 1853 and 1858 on the remains of a XVI century church. Inside it is possible to admire a painting of "Immacolata Concezione" by Pier Francesco Mazzucchelli (also known as the "Morazzone") and a valuable altarpiece by Bernardino Lanino.
- the Oratory of Santa Maria in Galnago, already mentioned in a document of 1347.
- the Romanesque style Oratory of Gaggiolo.
- Palazzo Bellini, in Neoclassical style.
- the Ethnographic Museum and the Museum of Religious Art