Ostøya is an island in the western section of Oslofjord (Vestfjorden), within the borders of the municipality of Bærum.

It is adjacent to the municipal border with Asker, where Nesøya and Brønnøya are to its northwest and southwest, respectively.

In 1791, the owner is listed as Hans Haagensen Tveter, and his descendants still own the main farm.

[1] In 1899 a significant portion of the island (Lille Oust) was sold for recreational summer houses.

[1] Two areas of the island have been set aside as nature reserves: Oustøen Country Club was privately established in 1965 and has about 700 members.