O.B.C. - Ontario Building Code

O.B.C.A. - Ontario Building Code Act

O. C. - On Center - The measurement of spacing for studs, rafters, and joists in a building from the center of one member to the center of the next.

O.D. - Abbreviation for Outside Diameter.

Oakum - Loose hemp or jute fiber that's impregnated with tar or pitch and used to caulk large seams or for packing plumbing pipe joints.

Occupancy - The use or intended use of a building or part thereof for the shelter or support of persons, animals or property.

Occupancy Load - The number of persons for which building or part thereof is designed.

Open Hole Inspection - When an engineer (or municipal inspector) inspects the open excavation and examines the earth to determine the type of foundation (caisson, footer, wall on ground, etc.) that should be installed in the hole.

Open Valley - A method of valley construction in which shingles on both sides of the valley are trained along a chalk line snapped on each side of the valley. Shingles do not extend across the valley. Valley flashing is exposed.

Oriel Window - An oriel window projects from the wall and does not extend to the ground. Oriel windows originated as a form of porch.

Oriented Strand Board (OSB) - Type of wood product used for sheathing, it is produced by gluing together three layers of wood chips or strands that are smaller than the chips used in wafer-board and are longer than they are wide. The chips in the top and bottom layers are parallel to the length of the panel and the chips in the center layer are perpendicular.

O-Ring - Round rubber washer used to create a watertight seal, chiefly around valve stems.

Orthographic Projection - A means of representing a three-dimensional (3D) object in two dimensions (2D).

Outdoor Air Supply - HVAC term for air brought into a building from the outdoors (often through the ventilation system) that has not been previously circulated through the system. Also known as "Make-Up Air".

Outrigger - An extension of a rafter beyond the wall line. Usually a smaller member nailed to a larger rafter to form a cornice or roof overhang.

Outside Corner - The point at which two walls form an external angle, one you usually can walk around.

Overhang - Outward projecting eave-soffit area of a roof; the part of the roof that hangs out or over the outside wall. See also Cornice.