The patron saint of Onzo is Saint Martin, celebrated on 11 November, with the parish church dedicated to him in the locality of Capitolo: in Baroque forms, with a simple façade adorned by four pinnacles, it holds inside two statues attributed to the Genoese sculptor Anton Maria Maragliano.
The earliest certain records of Onzo date from the year 1000, when the place was called Onze, deriving from the eleventh municipality of the county of Clavesana; having become the domain of Bonifacio del Vasto in 1091 and inherited by the Clavesana in the 12th century, it was ceded by them to the Republic of Genoa in 1393. Legend tells that the fief-holder Scipione Del Carretto, wishing to exercise the «ius primae noctis» over a newly married young woman, provoked the exasperation of the population, who stormed the castle by night, forcing him to flee and sacking the manor, partly demolished to recover building material: out of fear, the fief-holder sold the territory of Onzo to the Republic of Genoa, from which the population bought back the whole town and its lands. What remains today are the ruins of the castle of the marquises of Clavesana, with a polygonal plan and two truncated towers, one circular and one octagonal.
