Rather than build directly on the foundation of the medieval castle, architects Kazuyo Sejima and Ryue Nishizawa elevated the O-Museum above the ground. Visitors enter the lobby via a square ramp. A large window overlooks the castle remains. Because the O-Museum is elevated, its walls are protected from dampness rising from the earth.
The Ogasawara Museum is one of the earliest collaborations by architects Kazuyo Sejima and Ryue Nishizawa. The project begun in 1995 and was completed in 1999.