| Acronym | Meaning |
| OR | Operations Room |
| OR | Outer Roll |
| OR | Oxygen Relief |
| ORB | Orbit |
| ORB | Order |
| ORB | 1 -g Orbiter One-g (trainer) |
| ORCHIS | Oak Ridge Computerized Hierarchical Information |
| ORCHIS | System |
| ORD | Operational Readiness Date |
| ORD | Operational Ready Data |
| ORD | Operational Requirements Document |
| ORD | Orbital Requirements Document |
| ORD | Ordnance |
| ORDC | Orbiter Data Reduction Center |
| ORE | On-Orbit Repair Experiment |
| ORF | Orifice |
| ORG | Organization |
| ORI | Operational Readiness Inspection |
| ORIDE | Override |
| ORIEN | Orientation |
| ORIG | Origin |
| ORLA | OPtimum Repair Level Analysis |
| ORP | OFS Retransmission Processor |
| ORR | Operational Readiness Review |
| ORR | Operations Requirements Review |
| ORS | Orroval Valley |
| ORS | Orbital Refueling System |
| ORSDI | Oak Ridge Selective Dissemination Of Information |
| ORT | Operational Readiness Test |
| ORT | Orbit Readiness Test |
| ORTHOG | Orthogonal |
| OS | On-Orbit Station |
| OS | Operating Software |
| OS | Operating System |
| OS | OPtics Subsystem |
| OS | Orbiter CEI Specification |
| OSA | Operational Support Area |
| OSC | Oscillator |
| OSDH | Orbiter System Definition Handbook |
| OSE | Operating SupPort Equipment |
| OSE | Orbiter Support Equipment |
| OSF | Office Of Space Flight (now OSTS) |
| OSF | Ordnance Storage Facility |
| OSHA | Occupational Safety And Health Act (Activity) |
| OSHA | Occupational Safety And Health Administration |
| OSLM | Operations ShOP/Lab Manager |
| OSM | Orbital Service Module |
| OSMP | Operational SuPport Maintenance Plan |
| OSO | Ocean SYstems Operation |
| OSOP | Orbiter Systems Operating Procedures |