Onzo ( in Ligurian: Unsu ) is a municipality with 224 inhabitants (as at 31 December 2012) in the Italian region of Liguria. Politically it belongs to the province of Savona.
Onzo is located in the upper part of the Valle Arroscia to about 400 meters above sea level on the southern slope of Mount Peso Grande ( 1092 meter). The associated settlement Capitolo is, however, on a ridge between the valleys of the Rio Cornareo and Paraone. The municipality belongs to the Comunità Montana Ingauna and is approximately 64 km from the provincial capital Savona and about 20 kilometers from Albenga.
According to the Italian classifications for seismic activity, the community of Zone 3 was assigned. This means that Onzo is in a seismically active zone little.
The municipality is classified under climate category E, since the number of degree days has a value of 2229. This means that in Italy legally regulated heating season is between October 15 and April 15, for 14 hours each day.