| Abbreviation | Definition |
|---|---|
| OCUC | Occupational Center of Union County |
| OCUS | Ohio Credit Union System |
| ODAA | Oregon Dental Assistants Association |
| OEIU | Office Employees International Union |
| OFT | Ohio Federation of Teachers |
| OFT | Ojai Federation of Teachers, Local 2119 |
| OIT | Organización Internacional del Trabajo (International Labor Organization) |
| OIU | Old Ignatians' Union |
| OJEU | Official Journal of the European Union |
| OJT | on-the-job training |
| OLBEU | Ontario Liquor Boards Employees' Union (Canada) |
| ONEA | Organisation Nationale des Etudiants Algériens |
| OPC | Odua Peoples Congress |
| OPITO | Offshore Petroleum Industry Training Organization |
| OPPA | Ontario Provincial Police Association |
| OPPF | Omega Psi Phi Fraternity |
| OPSBA | Ontario Public School Boards' Association |
| OPSEU | Ontario Public Service Employee Union |
| OPSEU | Ontario Public Service Employees Union |
| OPU | Oodua Progressive Union |