O-ring: Round rubber washer or gasket that is compressed to create a watertight seal, typically in a compression fitting.
oakum: Loose hemp or jute fiber that is impregnated with tar or pitch and used to caulk large seams and for packing plumbing pipe joints.
observations: Those items of interest noted by an inspector during the walk-through survey portion of an inspection.
observe: To visually notice.
obvious: A condition or fact not likely to be ignored or overlooked.
occupancy load: The number of people permitted in a building based on square footage and the means of egress.
occupant: Any individual living in, sleeping in and/or having possession of a space within a building.
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 that permits the offsetting of a drainage run in the same direction.
OG: A molding with a profile in the form of a letter S; having the outline of a reversed curve.
ohm meter (ohmmeter): In electrical contracting, a device that measures the resistance across a load and used to track down broken wires. Never used on a live circuit.
Ohm’s Law: States that, in a given electrical circuit, the amount of 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 (resistance), or R = V ÷ I.
oil-canning: A term that describes the distortion of thin-gauge metal panels that are fastened in a manner that restricts normal thermal movement.
on center (OC): A term describing the measurement of the distance center to center between like materials, such as studs, rafters, joists, etc. For example, studs placed 16 inches OC are laid out so that there are 16 inches from the center of one stud to the center of the next stud.
open valley: Method of construction by which shingles on both sides of a valley are trimmed along a chalk line snapped on each side of the valley. Shingles do not extend across the valley, leaving the valley flashing exposed.
open-hole inspection: Describes an inspection performed by an engineer or municipal inspector to evaluate an open excavation and examine the earth to determine the type of foundation that should be installed in the hole (caisson, footer, wall on ground, etc.).
operate: To cause systems to function or turn on with normal operating controls.
operational: Systems or components capable of being safely operated.
optimum value engineering (OVE): Framing techniques that reduce unnecessary lumber use and improve the R-value of the wall by reducing thermal bridging and maximizing the wall area that is insulated.
oral consultation: A limited visual inspection of specific systems, structures and/or components of a property after which no written report is prepared by the inspector and s/he communicates his/her findings, opinions, conclusions and recommendations verbally to the client.
organic: A designation for any chemical compound that contains carbon and hydrogen.
organic felt: An asphalt roofing base material manufactured from cellulose fibers.
oriented strand board (OSB): A manufactured wood panel made of 1- to 2-inch wood chips and glue, often used as a substitute for plywood in exterior walls and roof sheathing. Also called chipboard, flakeboard andwaferboard.
outrigger: An extension of a rafter beyond the wall line. Usually, a smaller member is nailed to a larger rafter to form a cornice or roof overhang.
overhang: The part of a roof structure that extends horizontally beyond the vertical plane of the building’s exterior walls.
owner: Any person, agent, operator, firm or corporation having a legal or equitable interest in a property.
oxidize: The process of something combining with or being exposed to oxygen in the air; rust.