Ostøya is an island in the western section of Oslofjord (Vestfjorden), within the borders of the municipality of Bærum. It is the second largest island in the Oslo basin, after Nesøya. It is adjacent to the municipal border with Asker, where Nesøya and Brønnøya are to its northwest and southwest, respectively. Within Bærum, Borøya is to the north, Kjeholmen to the northeast, and Grimsøya to the east.