| | kthkxbye | | okay thanks bye |
| | mmmmlsbas | | olympic cities - munich to sydney |
| | mmmmlsbasc | | olympic cities - munich to china |
| | O.M.A.O. | | Office of Marine and Aircraft Operations |
| | O.M.G.T.T.H.L. | | oh my god to the highest level |
| | o.t.w. | | on the way |
| | o.u. | | organisational unit |
| | o.u.i. | | operating under the influence |
| | o.w.i.l. | | operating while intoxicated |
| | O/P | | out of print |
| | O/T | | overtime |
| | OA | | Office Administrator |
| | oamc | | once a month cooking |
| | oap | | old age pensioner |
| | oap | | old age person |
| | oap | | old aged pensioners |
| | oap | | old aged person |
| | oap | | old fart |
| | oap | | old guy |
| | oap | | old person |
| | oap | | old timer |
| | oap | | old-timer |
| | oap | | one act play |
| | oapn | | on a personal note |
| | OBE | | Order of the British Empire |
| | OBE | | Overcome By Events |
| | OBICU | | Obstetric Intensive Care Unit |
| | OBMU | | on-board management unit |
| | obo | | one by one |
| | obo | | or best offer |
|