| Acronym | Meaning |
| OA | Over Acting Over Achiever |
| OAN | On Another Note |
| OAO | Over And Out |
| OAP | Old Age Pensioner |
| OATUS | On A Totally Unrelated Subject |
| OAUS | On An Unrelated Subject |
| OB | Oh, Brother |
| OBBY | Oh, Baby |
| OBE | Overcome By Events Out of Body Experience Order of the British Empire |
| OBHWF | One Big Happy Weasely Family |
| OBO | Or Best Offer |
| OBQ | Oh, Be Quiet |
| OBTW | Oh, By The Way |
| OBX | Outer Banks North Carolina |
| OC | Of Course Original Character Orange County |
| OCD | Obsessive Compulsive Disorder |
| OCN | Of Course Not |
| OCO | Oh, Come On |
| OCR | Optical Character Recognition |
| OCT | On Company Time |
| ODAAT | One Day At A Time |
| ODF | Own Damn Fault |
| ODG | Oh Dear God |
| ODL | Oh Dear Lord |
| ODST | Orbital Drop Shock Trooper (Gaming) |
| ODT | On Dis Ting |
| OE | Outlook Express Operator Error |
| OED | Oxford English Dictionary |
| OEM | Original Equipment Manufacurer |
| OFAY | Offensive term for a white person |
| OFGS | Oh, For God’s Sake |
| OFH | Old Folks’ Home |
| OFOC | Overwhelming Feelings of Concern |
| OFWGKTA | Odd Future Wolf Gang Kill Them All (rappers) |
| OG | Original Gangster Old Git |
| OGIM | Oh God It’s Monday |
| OGK | Only God Knows |
| OGN | Online Gaming Network |
| OH | Other Half |
| OHAC | Own House And Car |
| OHIM | Oh Hell, It’s Monday |
| OHKO | One Hit Knock Out |
| OHN | Oh Hell, No! |
| OHS | Operator Head Space |
| OHT | One Handed Typing |
| OHW | One Hit Wonder |
| OJ | Only Joking |
| OJD | Obsessive Jonas (Brothers) Disorder |
| OJT | On Job Training |
| OLED | Organic Light-Emitting Diode |
| OLLG | One Less Lonely Girl |
| OLOY | Only Losers Obey Yolo |
| OLTL | One Life To Live |
| OM | Old Man |
| OMA | Oh My Allah |
| OMB | On My Break |
| OMD | Oh My Days |
| OMDB | Over My Dead Body |
| OMFSM | Oh My Flying Spaghetti Monster |
| OMFUG | Other Music Found UnderGround |
| OMG | Oh My God |
| OMGA | Oh My Giddy Aunt |
| OMGD | Oh My God Dude |
| OMGNA | Oh My Gosh, Not Again |
| OMJ | Oh My Jonas ( ) |
| OMJC | Oh My Jesus Christ |
| OML | Oh My Lord |
| OMS | Oh My Science |
| OMSJ | Oh My Sweet Jesus! |
| OMT | One More Thing |
| OMW | On My Way |
| OMWH | On My Way Home |
| OMZ | Oh My Zeus |
| ONM | Oh, Never Mind |
| ONNA | Oh No, Not Again |
| ONNTA | Oh No, Not This Again |
| ONO | Or Nearest Offer |
| ONS | One Night Stand |
| ONTD | Oh No They Didn’t |
| ONYD | Oh No You Didn’t |
| OOA | Out Of Ammo |
| OOAK | One Of A Kind |
| OOBE | Out Of Body Experience |
| OOC | Out Of Control/Character/Context |
| OOF | Out Of Office |
| OOH | Out Of Here |
| OOI | Out Of Interest |
| OOM | Out Of Mana ( ) |
| OOMF | One Of My Friends/Followers |
| OOML | Out Of My League |
| OOMM | Out Of My Mind |
| OOO | Out Of Office |
| OOP | Out Of Place |
| OOS | Out Of Style |
| OOSOOM | Out Of Sight, Out Of Mind |
| OOT | Out Of Town |
| OOTB | Out Of The Box |
| OOTC | Obligatory On-Topic Comment |
| OOTD | Offer Of The Day Outfit Of The Day |
| OOTO | Out Of The Office |
| OOTW | Out Of This World |
| OOYL | Out Of Your League |
| OOYM | Out Of Your Mind |
| OP | Original Poster |
| OPB | Other People’s Business |
| OPK | Other People’s Kids |
| OPM | Other People’s Money |
| OPPS | Misspelling of ‘oops’ |
| OSFA | One Size Fits All |
| OSLT | Or Something Like That |
| OSO | Other Significant Other |
| OSS | Open Source Software |
| OST | Original Sound Track |
| OT | Off Topic |
| OTA | Over The Air |
| OTB | Off The boat |
| OTC | Over The Counter |
| OTD | On The Dot Out The Door |
| OTE | Over The Edge |
| OTF | Off Topic Forum |
| OTFL | On The Floor Laughing |
| OTG | On The Go Off The Ground |
| OTH | Off The Hook |
| OTK | Over The Knee |
| OTLTA | One Thing Led To Another |
| OTM | Of The Moment One Track Mind |
| OTN | On That Note |
| OTO | One Time Only |
| OTOH | On The Other Hand |
| OTOOH | On The Other Other Hand |
| OTP | One True Pairing |
| OTR | On The Run |
| OTS | On The Side |
| OTT | Over The Top |
| OTTH | On The Third Hand |
| OTTOMH | Off The Top Of My Head |
| OTW | On The Way |
| OTW | On The Whole |
| OTY | Over To You |
| OU | Open University |
| OUAT | Once Upon A Time |
| OUO | Official Use Only |
| OVA | Original Video Animation |
| OVNO | Or Very Near Offer |
| OWK | Other World Kingdom |
| OWNM | Oh Well, Never Mind |
| OWOA | One Way Or Another |
| OWS | Occupy Wall Street |
| OWTTE | Or Words To That Effect |
| OY | Oh Yeah |
| OYB | On Your Bike |
| OYG | Oh Your God |
| OYO | On Your Own |