O. G. (or Ogee): A molding with a profile in the form of a letter S; having the outline of a reversed curve.

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

OD (Outside Diameter): A measurement of the diameter of a pipe as taken from the outside edge. A common method for sizing pipe.

Offset: A tubular component which permits the offsetting of a drainage run in the same basic direction.

Ohm's Law: States that, in a given electrical circuit, the amount at current in amps is equal to the pressure in volts divided by the resistance in ohms. The formula is: I (Current) = V voltage or V = I x R R resistance or R = V/I.

Ohmmeter: In electrical contracting, a device to measure the resistance across a load. They are never used on a live circuit. Used to track down broken wires.

Oil-Canning: The term describing distortion of thin-gauge metal panels which are fastened in a manner restricting normal thermal movement.

On Center (O.C.): A measurement term meaning a certain distance between like materials. Studs rafters, joists, and the like in a building placed at 16 inches O.C. will be laid out so that there is 16 inches from the center of one stud to the center of the next.

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: Method of valley construction in which shingles on both sides of the valley are trimmed along a chalk line snapped on each side of the valley. Shingles do not extend across the valley. Valley flashing is exposed.

Organic: A term designating any chemical compound which contains carbon and hydrogen.

Organic Felt: An asphalt roofing base material manufactured from cellulose fibers.

Oriented Strand Board (OSB, Chip Board, Wafer Board): A manufactured wood panel made out of 1"- 2" wood chips and glue. Often used as a substitute for plywood in the exterior wall and roof sheathing.

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.

Overhang: That part of the roof structure which extends horizontally beyond the vertical plane of the exterior walls of a building.

Oxidize: To combine with oxygen in the air.