| O | Opponent |
| OAO | Over And Out |
| OAUS | On An Unrelated Subject |
| OBE | Overcome By Events |
| OBTW | Oh By The Way |
| OBX | Old Battle Axe |
| OC | Original Character -or- Own Character |
| OCD | Obsessive Compulsive Disorder |
| ODTAA | One Damn Thing After Another |
| OIC | Oh, I See |
| OICU812 | Oh I See, You Ate One Too |
| OK | All Correct |
| OL | Old Lady |
| OLL | OnLine Love |
| OLN | OnLine Netiquette |
| OM | Old Man |
| OMB | Oh My Buddha |
| OMDB | Over My Dead Body |
| OMFG | Oh My F***ing God |
| OMG | Oh My God |
| OMIK | Open Mouth, Insert Keyboard |
| OML | Oh My Lord |
| ONID | Oh No I Didn't |
| ONNA | Oh No, Not Again |
| ONNTA | Oh No, Not This Again |
| ONUD | Oh No You Didn't -or- Oh No You Don't |
| OOAK | One Of A Kind |
| OOC | Out Of Character -or- Out Of Control |
| OOF | Out Of Facility |
| OOI | Out Of Interest |
| OOO | Out Of Office |
| OOTB | Out Of The Box -or- Out Of The Blue |
| OOTC | Obligatory On Topic Comment |
| OSIF | Oh Sh** I Forgot |
| OSINTOT | Oh Sh** I Never Thought Of That |
| OT | Off Topic |
| OTASOIC | Owing To A Slight Oversight In Construction |
| OTC | Over The Counter |
| OTF | Off The Floor -or- On The phone (Fone) |
| OTH | Off The Hook |
| OTL | Out To Lunch |
| OTOH | On The Other Hand |
| OTP | On The Phone |
| OTT | Over The Top |
| OTTOMH | Off The Top Of My Head |
| OTW | Off The Wall |
| OUSU | Oh, You Shut Up |
| OWTTE | Or Words To That Effect |
| OZ | stands for: Australia |
|
