| 項目 |
| O |
| O 脚［(ド) O-Beine］ |
| O3［ozone］ |
| OA1［office automation］ |
| OA2［Olympic Airways］ |
| OAG［Official Airline Guide］ |
| OAPEC（オアペック）［Organization of Arab Petroleum Exporting Countries］ |
| OAS［Organization of American States］ |
| OAU［Organization of African Unity］ |
| OB1［old boy］ |
| OB2［out-of-bounds］ |
| OCA［Olympic Council of Asia］ |
| OCI［overall commodity index］ |
| OCIA［Organic Crop Improvement Association］ |
| OCN［Open Computer Network］ |
| OCOG［Organizing Committee of Olympic Games］ |
| OCP［optional calling plan］ |
| OCR［optical character reader］ |
| OD1［orthostatic dysfunction］ |
| OD2［(和) over＋doctor］ |
| OD3［overdose; overdosage］ |
| ODA［Official Development Assistance］ |
| ODA 大綱 |
| Oe［oersted］ |
| OECD［Organization for Economic Cooperation and Development］ |
| OECD-NEA［OECD Nuclear Energy Agency］ |
| OECF［Overseas Economic Cooperation Fund］ |
| OED［Oxford English Dictionary］ |
| OEEC［Organization for European Economic Cooperation］ |
| OEIC［opto-electronic integrated circuit］ |
| OEM［original equipment manufacturing］ |
| OF ケーブル［oil-filled cable］ |
| Off-JT［off-the-job training］ |
| OG［(和) old＋girl］ |
| OGL［open general license］ |
| OH 通信［over-the-horizon］ |
| OHC エンジン［overhead camshaft engine］ |
| OHP［overhead projector］ |
| OHV［overhead valve］ |
| OIC1［Office of International Culture］ |
| OIC2［optical integrated circuit］ |
| OIC3［Organization of the Islamic Conference］ |
| OIT［Oita］ |
| OJT［on-the-job training］ |
| OK［oll korrect（all correct の発音綴），また Old Kinderhook Club からとも］ |
| OKA［Okinawa］ |
| OKJ［Okayama］ |
| OL1［(和) office＋lady］ |
| OL2［(ド) Orientierungslauf; (英) orienteering］ |
| OL3［overlap］ |
| OLE［object linking and embedding］ |
| OLTP［on-line transaction processing］ |
| OM［Sultanate of Oman］ |
| OMA1［object management architecture］ |
| OMA2［orderly marketing agreement］ |
| OMA3［Sultanate of Oman］ |
| OMB［Office of Management and Budget］ |
| OMR［optical mark reader］ |
| OMUNIMAX |
| ONA（オナ）［open network architecture］ |
| ONU［optical network unit］ |
| ONUMOZ［United Nations Operation in Mozambique］ |
| ONUSAL［United Nations Operation in El Salvador］ |
| OOC［Olympic Organizing Committee］ |
| OOP［object oriented programming］ |
| O.P.［out of print］ |
| Op アート［Optical Art］ |
| op.1［operation］ |
| op.2［(ラ) opus］ |
| OPC［organic photoconductor］ |
| OPCW［Organization for the Prevention of Chemical Weapons］ |
| OPEC（オペック）［Organization of Petroleum Exporting Countries］ |
| OPP［orthophenyl phenol］ |
| OPS［open profiling standard］ |
| Optacon（オプタコン）［optical-to-tactile converter］ |
| OR1［operating room］ |
| OR2［operations research］ |
| ORD［Chicago O'Hare］ |
| ORF［Norfolk］ |
| ORT［orthoptist］ |
| ORY［Orly］ |
| OS1［Austrian Airlines］ |
| OS2［operating system］ |
| OS, o/s3［out of stock］在庫切れ。品切れ。 |
| Os［osmium］ |
| OS/2 |
| OSA［Osaka］ |
| OSB［oriented strand board］ |
| OSCE［Organization for Security and Cooperation in Europe］ |
| OSI［Open Systems Interconnection］ |
| OSRD［Office of Scientific Research and Development］ |
| OSTP［Office of Science and Technology Policy］ |
| OT1［occupational therapist］ |
| OT2［overtime］ |
| O.T.［Old Testament］ |
| OTC［over-the-counter］ |
| OTDR［Optical Time Domain Reflectometers］ |
| OTE［on-target earnings］ |
| OTEC［ocean-thermal energy conversion］ |
| OTL［over the line］ |
| OTO［Office of Trade and Investment Ombudsman］ |
| O.U.1［Open University］ |
| O.U.2［Oxford University］ |
| O.U.P.［Oxford University Press］ |
| Oxfam（オックスファム）［Oxford Committee for Famine Relief］ |
| OZ［Asiana Airlines］ |
| oz.［(イ) onza; (英) ounce］ |
| OZMA（オズマ）計画 |