| Ω | Ohm |
| O3 | Ozone |
| OA | OFFICE AUTOMATION; Outdoor Air |
| OAQPS | Office of Air Quality Planning and Standards |
| OAR | Office of Air and Radiation |
| OBD Group | Onboard Diagnostics Group |
| OBE | Operating Basis Earthquake |
| OBM | Observationally Based Method |
| OCA | Other Cleanup Activity |
| OCC | Old Corrugated Containers |
| OCR | Office of Civil Rights; OPTICAL CHARACTER RECOGNITION |
| OCS | Outer Continental Shelf |
| OCS Source | Outer Continental Shelf Source |
| OD | OPTICAL DISK |
| ODMR | Optically Detected Magnetic Resonance |
| ODP | Ozone Depletion Potential |
| ODS | Ozone Depleting Substances |
| O/E Ratio of Taxa Loss | Observed/Expected Ratio of Taxa Loss |
| OECA | Office of Enforcement and Compliance Assurance |
| OEM | Original Engine Manufacturer; Original Equipment Manufacturer |
| OEP | Occupant Emergency Plan |
| OF | OPTIONAL FORM; Oxygenated Fuels |
| Offshore LNG Import Terminal | Offshore Liquefied Natural Gas Import Terminal |
| O&G | Oil and Grease |
| OGC | Office of General Council |
| OGI Camera | Optical Gas Imaging Camera |
| OGV | Ocean-Going Vessel |
| OH&S | Occupational Health and Safety |
| OHSAS 18001:2007 | Occupational Health and Safety Assessment Series 18001:2007 |
| OID | Outreach and Information Division |
| OLD Operation | Organic Liquids Distribution Operation |
| O&M | Operation and Maintenance; Operations and Maintenance |
| Ωm | Ohm-meter |
| OMMSQA | Office of Modeling, Monitoring Systems, and Quality Assurance |
| ON | Oligonucleotides |
| OP Years Old | Original Production Years Old |
| OPA | Oil Pollution Act |
| OPP | Office of Pesticide Programs |
| OPPT | Office of Pollution Prevention and Toxics |
| OPPTS | Office of Prevention, Pesticides and Toxic Substances |
| OPR | Ongoing Precision and Recovery Standard |
| OPSG | Ozone Policy & Strategies Group |
| OR | Odds Ratio |
| ORD | Office of Research and Development |
| Organic HAP | Organic Hazardous Air Pollutant |
| Organic HAP Content | Organic Hazardous Air Pollutant Content |
| Organic HAP Mass Fraction | Organic Hazardous Air Pollutant Mass Fraction |
| Organic HAP Overall Control Efficiency | Organic Hazardous Air Pollutant Overall Control Efficiency |
| Organic HAP Solvent | Organic Hazardous Air Pollutant Solvent |
| ORL | One Record per Line |
| ORM | Other Regulated Material |
| ORP | Oxidation-Reduction Potential |
| ORR | Oxidation-Reduction Reaction |
| OS | OPERATING SYSTEM |
| OSB | Oriented Strandboard |
| OSC | On-Scene Coordinator |
| OSCP | Office of Science Coordination and Policy |
| OSHA | Occupational Safety & Health Act; Occupational Safety and Health Act of 1970; Occupational Safety and Health Administration |
| OSHA PEL | Occupational Safety and Health Administration's Permissible Exposure Limit |
| OSP | Operator Service Provider |
| OSPS | Outreach and Special Projects Staff |
| OSRTI | Office of Superfund Remediation and Technology Innovation |
| OSW | Office of Solid Waste |
| OSWER | Office of Solid Waste and Emergency Response |
| OTAFM | Over The Air Folder Management |
| OTAQ | Office of Transportation and Air Quality |
| Other Nonferrous Foundry HAP | Other Nonferrous Foundry Hazardous Air Pollutant |
| OTR | Ozone Transport Region |
| OTSA Joint-Use Areas | Oklahoma Tribal Statistical Areas Joint-Use Areas |
| OU | Operable Unit |
| OVE | Optimum value engineering |
| Overall HAP Control Efficiency | Overall Hazardous Air Pollutant Control Efficiency |
| OWGLs | Open Waters of the Great Lakes |
| OWTS | Onsite Wastewater Treatment System |