Operable window: Window which can be opened for ventilation.

Operator: A metal arm and gear attached to the frame of a window and connection with a sash which allows easy operation.

Outer Frame Member: the exterior protruding portion of a window frame that has no exterior casing.

Outside Casing: That portion of the window frame which is exposed to the outdoors.

Obscure Glass: Glass that has been made translucent instead of transparent. Often used in bathrooms and decoration.

Oriel Window: An oriel window projects from the wall and does not extend to the ground.

Outside Glazing: Glazing installed from the outside.

Operating Panel: two door or two panel swing or sliding door, the panel that opens.