| "O" |
| O Joe |
| O.C. |
| O.M. Weenie |
| Oakley |
| Oatmeal |
| Obadiah |
| October |
| Oddball |
| Oddie |
| Oden |
| Odessa |
| Odie |
| Odie Pighet |
| Odis |
| Ofi |
| Ogden |
| Oggie |
| Oh Gee the Scott |
| OH NO |
| Ohana |
| Olaf |
| Old Man |
| Oleang |
| Olga |
| Oli |
| Olie | |
| Olik |
| Olive |
| Olive U |
| Oliver |
| Oliver Etouffee |
| Oliver James |
| Oliver Mo |
| Olivia |
| Olivia Cocoa |
| Olivia Grace |
| Olivia Newton Jon |
| Olivia Rose |
| Olivier Hilton |
| Ollie |
| Omar |
| Omelette |
| Omi |
| Ona |
| One Eyed Jack |
| Oneills Buffy Darling |
| Onika |
| Onix |
| Onni |
| Onslow |
| Onyx |
| OnyxOni | |
| Oola |
| Oolong |
| Oomp |
| Oona |
| Oopsie Daisey |
| Ooshky |
| Opal |
| Opey |
| Ophelia Badelia |
| Opi |
| Opie |
| Oppsie Daisy |
| Oprah |
| Optimus |
| Oral Saphrone |
| Orbie |
| Oreo |
| Oreo Bauer |
| Oreo Cookie |
| Orion |
| Orpheus Rex |
| Orso |
| Orville |
| Osacar |
| Oscar |
| Oscar Delarenta | |
| Oscar Mayer |
| Oscar Myer |
| Oscar Myers |
| Oscer |
| Oscor |
| Oshea |
| Osiris |
| Osito |
| Oskar |
| Osker Bane |
| Oso |
| Osten |
| Oswald |
| Otie |
| Otis |
| Otis Magoo |
| Otis Pedro |
| Ottie |
| Otto |
| Outlaw |
| Owen |
| Oz |
| Ozzie |
| Ozzy |
| Ozzy Singletary | |