| Label | Concept definition | Figure(s) | Code |
|---|---|---|---|
| OASIS | A small, isolated, fertile, or green <TRACT> in a <DESERT_REGION> usually having a <WATERING_PLACE>; oasis. | 817 | |
| OBJECT | A person, place, thing, or concept; object. | 818 | |
| OBJECT_PROPERTY_SET | A <PROPERTY_SET> describing an <OBJECT>. | 819 | |
| OBJECT_SET | A <NON_EMPTY_SET> of <OBJECT>s. | 820 | |
| OBSERVATION_STATION | A station that is equipped for observation and study. | 821 | |
| OBSERVATORY | An <OBSERVATION_STATION> designed and equipped for making observations of astronomical, meteorological, or other natural phenomena; observatory [SOED, "observatory", 1]. | 822 | |
| OCCLUDED_FRONT | A composite of two <ATMOSPHERIC_FRONT>s formed as a <COLD_FRONT> overtakes a <WARM_FRONT> or a <STATIONARY_FRONT>; occluded front. | 823 | |
| OCCUPATION_ZONE | A <REGION> temporarily held and controlled by a hostile military force; occupation zone. | 824 | |
| OCEAN | One of the major divisions of an expanse of salt <WATER> covering a <PLANETARY_SURFACE>; ocean. | 825 | |
| OCEAN_BASIN | A <TERRAIN_DEPRESSION> of an <OCEAN_FLOOR> approximately equidimensional in plan view and of variable extent; ocean basin. | 826 | |
| OCEAN_FLOOR | The <WATERBODY_FLOOR> of an <OCEAN>. | 827 | |
| OCEAN_FLOOR_TOPOGRAPHIC- _CONFIGURATION |
A <WATERBODY_FLOOR_TOPOGRAPHIC_CONFIGURATION> of an <OCEAN_FLOOR>. | 1440 | |
| OFF_ROUTE_BEARING | The <DIRECTION> and distance from an <AID_TO_NAVIGATION> to a defined <WAYPOINT>; off route bearing. | 828 | |
| OFFSHORE_LOADING_FACILITY | A <FACILITY> located in a <WATERBODY> for loading and unloading <CARGO>; offshore loading facility. | 829 | |
| OFFSHORE_PLATFORM | A <STRUCTURE> placed in a <WATERBODY> and used: for production, loading, and discharge; as an <OBSERVATION_STATION>; or as a <SCIENCE_FACILITY>; offshore platform. | 830 | |
| OIL_BARRIER | A floating <BARRIER> to stop and contain oil flow on a <WATERBODY_SURFACE>; oil barrier. | 831 | |
| OIL_DRILLING_RIG | A <RIG> assembled primarily for the purpose of drilling an oil <WELL>; oil drilling rig. | 832 | |
| OPEN_AIR_BATH | An open air bath. | 833 | |
| OPEN_WATER | A <WATERBODY_REGION> where the <WATER> is flowing or free standing; open water. | 834 | |
| OPERA_HOUSE | A <BUILDING> that houses a <THEATRE> designed chiefly for the performance of operas; opera house. | 835 | |
| ORCHARD | A <TREED_TRACT> devoted to the cultivation of fruit or nut <TREE>s; orchard [AHD, "orchard", 1]. | 836 | |
| ORE_REFINERY | An <INDUSTRIAL_WORKS> for purifying ores (for example: copper and/or silver) and extracting from them various metal and chemical products; ore refinery. | 837 | |
| ORNAMENT | A material <OBJECT> serving to adorn, beautify, or embellish; ornament [SOED, "ornament", 2.a]. An ornament is also known as a decoration. | 1436 | |
| OUTBUILDING | A small <BUILDING> that is separate from but is located near a main <BUILDING>; outbuilding. | 838 | |
| OVERFALL_REGION | A <WATERBODY_REGION> where short, breaking <WATER_WAVE>s are formed on the <WATERBODY_SURFACE> when a strong <WATER_CURRENT> passes over a <SHOAL> or other submarine obstruction or meets a contrary <WATER_CURRENT> or <WIND>; overfall region. | 839 | |
| OVERHEAD_OBSTRUCTION | An overhead obstruction (for example: an underpass, an overhead <PIPELINE>, and/or a <BUILDING_OVERHANG>) on a <TRANSPORTATION_ROUTE>. | 840 | |
| OVERPASS | A <BRIDGE> used to provide clearance to traffic on a <LAND_TRANSPORTATION_ROUTE> at a lower elevation; overpass. | 841 |
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