| OSHA | Occupational Safety and Health Administration |
| oceanside |
| offgas |
| AEOD | Office for the Analysis and Evaluation of Operational Data (NRC) |
| OES (also ES) | Office of Emergency Services (State) |
| OIE or IE | Office of Inspection and Enforcement (State) |
| IG | Office of Inspector General (NRC) |
| NMSS | Office of Nuclear Material Safety and Safeguards (NRC) |
| off-line (adj), off line (adv) |
| offshore |
| offsite |
| andOmega; | ohm |
| OCB | oil circuit breaker |
| OWS | oil-water separator |
| one-dimensional |
| one-half |
| ongoing |
| on-line (adj), on line (adv) |
| onshore |
| onsite |
| ODES | Onsite Dosimetry and Environmental Services |
| OMMS | Onsite Meteorological Monitoring System |
| OTSC | Onsite Technical Support Center |
| OTSC | on-the-spot-change |
| OE | operability evaluation |
| OBA | operating basis accident |
| OBE | operating basis earthquake |
| OBI | operating basis incident |
| OEA | operating experience assessment |
| OL | operating license |
| OP | operating procedure |
| OVID | operating valve identification diagram |
| ORAT | Operational Readiness Assessment Team (NRC) |
| OSC | Operational Support Center |
| O&M | operations and maintenance |
| OOS | out of service |
| OD | outside diameter |
| ODSCC | outside diameter stress corrosion cracking |
| overcurrent |
| overemphasize |
| overpower |
| overpressurization |
| oversized |
| OPC | overspeed protection control |
| overtemperature |
| overvoltage |
