The patrons of Oleggio are the apostles Peter and Paul, celebrated on 29 June. Their church stands at the highest point of the town: designed in neoclassical style by Alessandro Antonelli, it was built between 1853 and 1858 over the earlier 16th-century church, with a basilican plan, three naves closed by large semicircular apses and a portico of four columns.
Before it, the parish was the pieve of San Michele, already recorded in 973, raised to pieve in 1133 and named as parish church in 1347, until in the 16th century it was abandoned: it is still there, inside the cemetery, Romanesque, with three naves and three apses and a crypt beneath the raised presbytery.
