- obelisk A slender monolithic shaft with a square perimeter and a pyramidal cap.
- object Real or virtual entities which can be individually identified.
- object naming Various versions of recognized text identifiers for an object.
- observation A place used primarily as a viewpoint with a vantage over a city, landscape, or activity.
- observation Data sources received as first-hand sensory input by the person entering the data.
- observation area closes permanently A room or deck for high observation is closed to the public, without prospect for re-opening.
- observation area opens A public room or deck for high observation opens to the general public.
- observation deck height The distance from base level to the highest outdoor public viewing area.
- observation floor height The distance from base level to the highest indoor public viewing area.
- observation ride A structure fixed to the ground with moving cabins or platforms that give riders an enjoyable perspective.
- observation tower A tower designed to give people high views over the surrounding area.
- observatory A site for telescopic or other observation and visual exploration of outer space.
- occupancy permit issued A legal certificate is issued by the government allowing occupancy of a structure to begin.
- occupancy zone A section of a structure occupied by a single tenant or related group of tenants.
- occupational safety and health A firm which ensures compliance with safety and health laws at a building site.
- oceanic shipping route A navigable water route across oceans and/or seas.
- ochre A dull, brownish shade of orange.
- octagon A roughly even 8-sided floor plan.
- octopus ride An amusement ride with multiple rings of cabs, each attached to a different rotating arm.
- office Places for desk-based work where people administer any kind of activities.
- office area Total open floor area used for office space.
- office building A building used for desk-based businesses.
- office condominium Non-industrial working space divided into separate owner-occupied units.
- office supply store A retail site specializing in office products and equipment, such as paper, ink, and office electronics.
- official dedication A ceremony or declaration for the opening of a structure is held by its owner, developer, or manager.
- official legal name The complete official name of an entity as registered with the government.
- official records Printed or written documents which were clearly produced by an authoritative source.
- official website The primary URL pertaining to an object and sanctioned by the controlling entity of that object.
- official website An internet page maintained by a person or organization with a professional involvement in the data topic.
- oil blue An extremely dark blackish shade of blue or very dark teal.
- oil distribution A fixed facility used in the movement of petroleum from its point of extraction to a refinery or point of sale.
- oil extraction A facility for extracting petroleum from the earth.
- oil heating Heat is generated by burning petroleum or other oil products on the site.
- oil-burning power plant A facility for generating electric power by burning petroleum fuel.
- old catholic church A house of worship in a branch of the Catholic faith which has broken with Roman authority.
- olive A dull, medium-to-dark shade of brownish green.
- oncology Clinical space for medical care and diagnosis of persons with cancer.
- onion dome A roof cap with a round base, pointed top, and bulging profile.
- only small pets allowed No large domestic animals are allowed to enter or to be kept on the premises.
- open angle A floor plan with 2 wings meeting at an angle.
- open floor plan Column-free space in large units or suites of office and industrial space.
- open framework A roof structure dominated by an open latticework of beams or tubes.
- open pit An excavated hole in the earth bounded on all sides and fully open to the atmosphere.
- open-air ground floor More than 50% of a building's footprint is unenclosed at the ground level.
- open-air structure A building or shelter where most of the interior space is not fully enclosed.
- openable windows Windows which can be opened to let outside air circulate inside.
- opening hours The scheduled times when a particular usage is made accessible.
- opera house A venue for live performances of operas before an audience.
- operational wind resistance The wind speed at which a structure must cease operations or evacuate occupants.
- ophthalmology Clinical space where doctors treat patients for conditions affecting the eyes.
- optometry Clinical space where doctors measure the vision of patients and prescribe corrections.
- orange Colors with a orangish hue, ranging from light peach to very dark orange or vermillion.
- orange Medium orange, about halfway between yellow and red.
- orchard A distinct area of intentionally planted trees intended for harvesting.
- orchid A light but not too pale shade of purple.
- ore storage Space for bulk storage of raw metal-bearing stone extracted from the earth.
- organ builder The company or workshop who builds a permanently installed musical organ.
- organic A load-bearing frame composed of biologically grown materials.
- organic material Facade materials taken from naturally grown organisms and retaining a more or less natural appearance.
- organic pattern Different shades organized in biomorphic layout with curving or irregular lines.
- oriel A cantilevered projecting bay with windows, which increases natural light and floor space inside a building.
- oriental orthodox church A house of worship in a faith belonging to Oriental Orthodoxy.
- oriental revival Traditional East Asian architectural forms applied to modern and postmodern designs.
- ornament Used primarily for purposes of decoration or aesthetics.
- ornamental feature Design elements added to a structure for aesthetic or stylistic purposes.
- ornamental illumination Major decorative lighting on the facade other than floodlighting or logos.
- ornamental pattern A multicolor surface largely filled with regularly arranged decorative flourishes.
- ornamental rail A building summit formed by a decorative metal fence.
- orthodox synagogue A house of prayer and worship in the orthodox Jewish faith.
- orthopedics Clinical space for medical treatment of broken and misaligned bones.
- osteopathic clinic A center for treatment of patients through osteopathic therapy, involving exterior manipulations of the body.
- otolaryngology A clinic or doctor's office specializing in care of nose, ear, and throat ailments.
- outdoor dining Tables and seating available for restaurant-style eating outdoors, often on a seasonal basis.
- outdoor gear store Retail space specializing in the equipment used in wilderness activities such as camping, hiking, and mountaineering.
- outdoor observation deck A public outdoor observation area on a high floor.
- outdoor stadium A structure with raked seating around a playing field without a full roof covering.
- outdoor station Any combination of platforms, stairs, shelters, and other elements forming an open-air transit station.
- outdoor theater An open-air site or structure with a stage and audience space.
- outdoor window screens Screens or blinds which cover the exterior sides of windows.
- outerwear store Retail space specializing in coats and protective clothing for cold or rainy weather.
- outlet mall A group of stores where manufacturers sell their wares directly to consumers, usually at a discount.
- outside stairs A set of steps built over sloping ground or over a precipice, which do not belong to any other structure.
- overflow storage Space for storing miscellaneous items in order to save space in living or working areas.
- overhanging water tower A tank resting atop a thick shaft, and overhanging the shaft by less than half its radius.
- overnight guest suites Designated rooms or units in a multi-unit residential building where guests of residents can spend the night.
- owner A legal property holder or shareholder of any object.
- owner of landed property The owner of the land on which a built object is placed, independently of the object itself.
- owner's broker commission The service fee paid by a property owner to an agent for a successful sale or lease transaction.
- owner's surrogate An agent who executes a property owner's responsibilities and acts on the owner's behalf.
- owner-occupied home A residential space occupied by its owner and free of membership in a condominium association.
- owner-occupied office Business workplaces held under a single owner who also occupies the space.