| O | Ohio (USA) |
| OAS | Organization of American States; On Active Service |
| OBE | Officer, Order of the British Empire |
| OC | Officer Commanding; Order of Canada (equivalent to former award SM) |
| OCS | Officer Candidates School |
| OCTU | Officer Cadet Training Unit |
| ODA | Overseas Development Administration |
| ODI | Overseas Development Institute |
| ODM | Ministry of Overseas Development |
| OEA | Old Etonian Association; Overseas Education Association |
| OECD | Organization for Economic Co-operation and Development |
| OED | Oxford English Dictionary |
| OEEC | Organization for European Economic Co-operation |
| OF | Order of the Founder, Salvation Army |
| Offr | Officer |
| OFS | Orange Free State |
| OH | Ohio |
| OK | Oklahoma |
| OLJ | Officer, Order of St Lazarus of Jerusalem |
| OM | Order of Merit |
| ON | Order of the Nation (Jamaica) |
| Ont | Ontario |
| OP | Ordinis Praedicatorum (of the Order of Preachers (Dominican)); Observation Post |
| Oppn | Opposition |
| op(s) | operation(s) |
| OR | Order of Rorima (Guyana); Operational Research; Oregon |
| ORT | Organization for Rehabilitation through Training |
| OSB | Order of St Benedict (Benedictine) |
| OSCE | Organisation for Security and Co-operation in Europe |
| OStJ | Officer, Most Venerable Order of the Hospital of St John of Jerusalem |
| OT | Old Testament |
| OTC | Officers' Training Corps |
| OU | Open University |
| OUDS | Oxford University Dramatic Society |
| OUP | Oxford University Press; Official Unionist Party |
| Oxon | Oxfordshire; Oxoniensis (of Oxford) |