| OA | Office of Administration |
| OAAG | Office of the Associate Attorney General |
| OAC | Office of Antiboycott Compliance |
| OACU | Office of Animal Care and Use |
| OAG | Office of the Attorney General |
| OAHP | Office of Affordable Housing Preservation |
| OAIT | Office of American Indian Trust (Archive) |
| OALJ | Office of Administrative Law Judges |
| OAQPS | Office of Air Quality Planning and Standards |
| OAR | Office of AIDS Research | Office of Oceanic and Atmospheric Research |
| OARM | Office of Attorney Recruitment and Management |
| OAS | Office of Applied Studies | Organization of American States |
| OASAM | Office of the Assistant Secretary for Administration and Management |
| OASDI | Old-Age and Survivors Insurance and Disability Insurance |
| OAT | Office for the Advancement of Telehealth |
| OBA | Office of Biotechnology Activities |
| OBEMLA | Office of Bilingual Education and Minority Languages Affairs (changed to OELA) |
| OBI | Office of Business Innovations |
| OBL | Office of Business Liaison |
| OBO | Bureau of Overseas Building Operations |
| OBPA | Office of Budget and Program Analysis |
| OBSSR | Office of Behavioral and Social Sciences Research |
| OCC | Comptroller of the Currency |
| OCDETF | Organized Crime Drug Enforcement Task Force |
| OCE | Office of the Chief Economist | Office of Congressional Ethics |
| OCFT | Office of Child Labor, Forced Labor, and Human Trafficking |
| OCIA | Office of Congressional and Intergovernmental Affairs |
| OCIO | Office of the Chief Information Officer |
| OCR | Office for Civil Rights (Dept. of Education) |
| OCRM | Ocean and Coastal Resource Management |
| OCRS | Organized Crime and Racketeering Section |
| OCRWM | Office of Civilian Radioactive Waste Management |
| OCS | Office of Coast Survey | Office of Community Services |
| OCSC | Office of Citizen Services and Communications (changed to OCSIT) |
| OCSE | Office of Child Support Enforcement |
| OCSIT | Office of Citizen Services and Innovative Technologies |
| OCSPP | Office of Chemical Safety and Pollution Prevention |
| OCSQ | Office of Clinical Standards and Quality |
| OD | Office on Disability |
| ODAG | Office of the Deputy Attorney General |
| ODEP | Office of Disability Employment Policy |
| ODGMO | Office of Departmental Grants Management and Oversight |
| ODLS | Online Detainee Locator System |
| ODNI | Office of the Director of National Intelligence |
| ODP | Office for Domestic Preparedness (merged into FEMA) | Office of Disease Prevention |
| ODPHP | Office of Disease Prevention and Health Promotion |
| ODR | Office of Dispute Resolution |
| OE | Office of Electricity Delivery and Energy Reliability |
| OEA | Office of Economic Adjustment |
| OECA | Office of Enforcement and Compliance Assurance |
| OECD | Organization for Economic Cooperation and Development |
| OEGT | Office of Electronic Government and Technology (changed to egov) |
| OEI | Office of Environmental Information |
| OELA | Office of English Language Acquisition |
| OEMM | Offshore Energy and Minerals Management |
| OEO | Office of Enforcement Operations |
| OERI | Office of Educational Research and Improvement (archive; see IES) |
| OERR | Office of Emergency Response and Recovery |
| OES | Bureau of Oceans and International Environmental and Scientific Affairs |
| OESE | Office of Elementary and Secondary Education |
| OET | Office of Educational Technology |
| OFA | Office of Family Assistance |
| OFAC | Office of Foreign Assets Control |
| OFAM | Office of Financial and Administrative Management |
| OFCCP | Office of Federal Contract Compliance Programs |
| OFCM | Office of the Federal Coordinator for Meteorology |
| OFDT | Office of the Federal Detention Trustee |
| OFHEO | Office of Federal Housing Enterprise Oversight (merged into FHFA) |
| OFM | Office of Financial Management |
| OFR | Office of the Federal Register |
| OGAC | Office of the U.S. Global AIDS Coordinator |
| OGC | White House Office of Global Communications |
| OGE | Office of Government Ethics |
| OGHA | Office of Global Health Affairs |
| OGIS | Office of Government Information Services (.pdf) |
| OGP | Office of Governmentwide Policy |
| OGWDW | Office of Ground Water and Drinking Water |
| OHAP | Office of HIV/AIDS Policy |
| OHASIS | Office of Health and Safety Information System |
| OHCHR | Office of the UN High Commissioner for Human Rights |
| OHHLHC | Office of Healthy Homes and Lead Hazard Control |
| OHPI | Office of Highway Policy Information |
| OHRP | Office for Human Research Protections |
| OHS | Office of Head Start |
| OHTA | Office of Historical Trust Accounting |
| OIA | Office of Insular Affairs | Office of International Affairs |
| OIE | World Organization for Animal Health |
| OIEP | Office of Indian Education Programs (changed to BIE) |
| OII | Office of Innovation and Improvement |
| OIIA | Office of Intergovernmental and Interagency Affairs (archive) |
| OIG | Office of the Inspector General (DOJ) | (USDA) |
| OIP | Office of Information and Privacy |
| OIPL | Office of Intergovernmental and Public Liaison |
| OIPR | Office of Intelligence Policy and Review |
| OIR | Office of International Relations |
| OIRA | Office of Information and Regulatory Affairs |
| OIT | Office of International Trade |
| OITA | Office of International and Tribal Affairs |
| OJC | Office of Job Corps |
| OJJDP | Office of Juvenile Justice and Delinquency Prevention |
| OJP | Office of Justice Programs |
| OLA | Office of Legislative Affairs |
| OLAW | Office of Laboratory Animal Welfare |
| OLC | Office of Legal Counsel |
| OLMS | Office of Labor Management Standards |
| OLP | Office of Legal Policy |
| OLR | Office of Labor Relations |
| OMAO | Office of Marine and Aviation Operations |
| OMAR | Office of Medical Applications of Research |
| OMB | Office of Management and Budget |
| OME | Office of Migrant Education |
| OMEI | Office of Minority Economic Impact |
| OMH | Office of Minority Health |
| OMHAR | Office of Multifamily Housing Assistance Restructuring (replaced by OAHP) |
| OMHD | Office of Minority Health and Health Disparities |
| OMHR | Office of Minority Health and Research |
| OMM | Offshore Minerals Management Program |
| ONAP | Office of National AIDS Policy | Office of Native American Programs |
| ONC | Office of the National Coordinator for Health Information Technology |
| ONCIX | Office of the National Counterintelligence Executive |
| ONDCP | Office of National Drug Control Policy |
| ONHIR | Office of Navajo and Hopi Indian Relocation |
| ONPE | Office of Non-Public Education |
| ONR | Office of Naval Research |
| ONS | Office of Nursing Services |
| OOHLHC | Office of Healthy Homes and Lead Hazard Control |
| OPA | Office of Population Affairs |
| OPAD | Office of Policy and Development |
| OPC | Office of the Police Corps (defunct; see Wayback Machine) |
| OPCL | Office of Privacy and Civil Liberties |
| OPDAT | Office of Overseas Prosecutorial Development, Assistance and Training |
| OPDR | Office of Policy Development and Research |
| OPE | Office of Postsecondary Education | Office of Public Engagement |
| OPEC | Organization of Petroleum Exporting Countries |
| OPEPD | Office of Planning, Evaluation and Policy Development |
| OPHS | Office of Public Health and Science |
| OPIC | Overseas Private Investment Corporation |
| OPL | Office of Policy and Legislation |
| OPL-IGA | Office of Public Liaison & Intergovernmental Affairs (changed to OPE) |
| OPM | Office of Personnel Management |
| OPPTS | Office of Prevention, Pesticides, and Toxic Substances (changed to OCSPP) |
| OPR | Office of Professional Responsibility |
| OPS | Office of Pipeline Safety |
| OPSEC | Operations Security |
| OPTF | Obscenity Prosecution Task Force |
| ORA | Office of Regulatory Affairs | Office of Research and Applications (changed to STAR) |
| ORACBA | Office of Risk Assessment and Cost-Benefit Analysis |
| ORD | Office of Rare Diseases |
| ORHP | Office of Rural Health Policy |
| ORI | Office of Research Integrity |
| ORISE | Oak Ridge Institute for Science and Education |
| ORMH | Office of Research on Minority Health (changed to OMHR) |
| ORNL | Oak Ridge National Laboratory |
| ORR | Office of Refugee Resettlement | Office of Response and Restoration |
| ORWH | Office of Research on Women's Health |
| OSAC | Overseas Security Advisory Council |
| OSBP | Office of Small Business Programs (see OSDBU) |
| OSC | Office of Special Counsel | Open Source Center |
| OSCE | Office of Child Support Enforcement |
| OSD | Office of the Secretary of Defense |
| OSDBU | Office of Small and Disadvantaged Business Utilization |
| OSDFS | Office of Safe and Drug-Free Schools |
| OSEC | Office of the Secretary (Dept. of Labor) |
| OSEP | Office of Special Education Programs |
| OSERS | Office of Special Education and Rehabilitative Services |
| OSF | Office of Space Flight (changed to Office of Space Operations) |
| OSFAP | Office of Student Financial Assistance Programs (changed to FSA) |
| OSG | Office of the Solicitor General | Office of the Surgeon General |
| OSHA | Occupational Safety and Health Administration |
| OSHRC | Occupational Safety and Health Review Commission |
| OSI | Office of Special Investigations |
| OSM | Office of Surface Mining Reclamation and Enforcment | Office of Spectrum Management |
| OSO | Office of Space Operations |
| OSS | Office of Strategic Services |
| OST | Office of the Special Trustee for American Indians |
| OSTI | Office of Scientific and Technical Information |
| OSTP | Office of Science and Technology Policy |
| OSWER | Office of Solid Waste and Emergency Response |
| OTA | Office of Technology Assessment |
| OTIA | Office of Telecommunications and Information Applications |
| OTJ | Office of Tribal Justice |
| OTLA | Office of Trade and Labor Affairs |
| OTP | Office of Technology Policy (unable to verify 4/09; see Wayback Machine) |
| OTS | Office of Thrift Supervision |
| OTT | Office of Transportation Technologies (changed to FreedomCAR) |
| OVAE | Office of Vocational and Adult Education |
| OVBD | Office of Veterans Business Development |
| OVC | Office for Victims of Crime |
| OVP | Office of the Vice President |
| OVW | Office on Violence Against Women |
| OWBO | Office of Women's Business Ownership |
| OWCP | Office of Workers' Compensation Programs |
| OWH | Office on Women's Health |
| OWOW | Office of Wetlands, Oceans and Watersheds |
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