| OBLIGATE | Invariably. A species which is unable to live outside of a given environment. Applied to animals living in the dark zone of caves that may display Troglomorphic adaptions. Biology - Flora & Fauna. Ref AC |
| ON BELAY | Signal given to a climber by the belayer, that the manually operated belay system is ready for use. Climbing and SRT. Ref MM |
| OOLITE | Oolitic limestone. A type of rock, often calcareous which grows by radial or concentric means around a nucleus. The term Oolite formerly referred to cave pearls. Karst - Kinds of. Ref JJ |
| OPEN TRAVERSE | A survey traverse which does not form a loop. Survey & Mapping term. Ref JJ |
| ORDER | 4th rank in Taxonomic system. Kingdom, Phylum, Class, Order, Family, Genus, Species. Biology - Flora & Fauna. Ref MM |
| ORGANIC | Of biological origin. (Strictly includes all carbon compounds, some of which are NOT biological). Biology - Flora & Fauna. Ref AC |
| ORIENTATION | The relationship of a survey line to true, grid or magnetic north. Survey & Mapping term. Ref JJ |
| OROGEN | Oro = mountain. A belt of deformed rocks, making up a portion of the continental crust. Geology. Ref MM |
| OUTFLOW CAVE | A cave from which a stream flows or formerly did, and which can not be followed upstream to the surface. Cave - Kinds of. Ref JJ |
| OVERHANG | A simple cave or rock shelter in which no part is in the dark zone. Cave - Kinds of. Ref MM |
| OVERHANG | A ladder or rope that hangs over a ledge or shelf of rock that projects past the rest of the rock face below, & thereby hangs free. Climbing and SRT. Ref MM |