| 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) |
