| Abbreviation | Definition |
|---|---|
| ORI | Operational Readiness Inspection |
| ORIDE | Override |
| ORIEN | Orientation |
| ORIG | Origin |
| ORMR | Orbiter Rollout Milestone Review |
| ORP | OFS (Orbiter Functional Simulator) Retransmission Processor |
| ORPA | Operational Readiness and Performance Assurance |
| ORPB | Oxygen Rich Pre-Burner |
| ORR | Operational Readiness Review |
| ORR | Operations Requirements Review |
| ORS | Orbital Refueling System |
| ORS | Orroval Valley, Australia (Spaceflight Tracking and Data Network station) |
| ORSDI | Oak Ridge Selective Dissemination of Information |
| ORT | Operational Readiness Test |
| ORT | Orbit Readiness Test |
| ORTHOG | ORTHOGonal |
| ORU | Orbital Replacement Unit |
| OS | On-orbit Station |
| OS | Operating Software |
| OS | Optics Subsystem |