| Abbreviation | Definition |
|---|---|
| OMA | Oh My Allah |
| OMAG | Oh My Atheist God |
| OMF | Overseas Missionary Fellowship |
| OMFG | Oh, My Fine God |
| OMG | Our Mighty God |
| OMI | Oblates of Mary Immaculate |
| OMSTAIN | One More Sign The Apocalypse Is Nigh |
| OMY | Online Muslim Youth |
| ONA | Order Of Nine Angels |
| OOCA | Online Orthodox Christian Assembly |
| OOF | Order Of Faith |
| OOG | Out Of Grace |
| OOL | Origin Of Life |
| OP | One Power |
| OP | Order of Preachers |
| OPC | Only Perfect Church |
| OPCOM | Old Plank Christian Organization of Men |
| OPF | Orthodox Peace Fellowship |
| OPM | Opening Peoples' Minds |
| OPN | Ora Pro Nobis (Latin: Pray For Us) |