Osterøy is an island municipality in the county of Hordaland, Norway. It is located in the traditional district of Nordhordland. The administrative centre is located in Lonevåg in the central part of the island, while the settlement with the largest population is Valestrandfossen with 1,012 inhabitants as of 1 January 2008.
Osterøy municipality covers the majority of the island of the same name, although the mostly uninhabited north-eastern part is part of Vaksdal municipality. Osterøy is located just northeast of Bergen, and is surrounded by the fjords Osterfjorden and Sørfjorden. The 19th century musician and composer Ole Bull had a home on the island at Valestrandfossen. The Havrå farm is located on the island.
Osterøy municipality was created on 1 January 1964. Part of Haus (population: 2,327), the Bruvikbygda area of Bruvik (population: 409), part of the municipality of Hosanger (population: 1,616), and part of the municipality of Hamre (population: 1,166) were merged to form the new municipality of Osterøy.
The Old Norse form of the name was "Óstr". The last element "øy" which means "island" was added later. The meaning of the name is unknown. It is possible that it comes from the word "óstr" which means "the arch of the neck"--words for parts of the body are common in Norwegian place names.