| Organic compound | (Environmental Engineering) Any compound containing carbon except for the carbonates (carbon dioxide, the carbonates and bicarbonates), the cyanides, and cyanates. |
| Organic nitrogen | (Environmental Engineering) Nitrogen contained as amines in organic compounds such as amino acids and proteins. |
| Oxidative phosphorylation | (Environmental Engineering) The synthesis of the energy storage compound adenosine triphosphate (ATP) from adenosine diphosphate (ADP) using a chemical substrate and molecular oxygen. |
| Object-oriented - | (Software Engineering) an approach to software development that makes use of a classification approach and packages data and processing together |
| Object-oriented analysis (OOA) - | (Software Engineering) a technique for defined classes of objects, their relationships and basic structure |
| Object-oriented design (OOD) - | (Software Engineering) a technique for translating the OOA model into an implementation model |
| Objects - | (Software Engineering) a named element of the problem domain containing data and processing |
| OCL (Object Constraint Language) - | (Software Engineering) a supplement to UML, this formal language allows a software engineer to construct unambiguous statements about the characteristics of various design model elements |
| Outsourcing - | (Software Engineering) contracting software work to a third party |
| Ottawa Sand | (Concrete Engineering) A sand used as a standard in testing hydraulic cements by means of mortar test specimens. Sand is produced by processing silica rock particles obtained by hydraulic mining of the orthoquartzite situated in open-pit deposits near Ottawa, Illinois; naturally rounded grains of nearly pure quartz. |