| odd | Prelude |
| oldGenFactor | GHC.RTS.Flags |
| on | Data.Function |
| OneLineGCStats | GHC.RTS.Flags |
| OneShot | GHC.Event |
| onException | Control.Exception.Base, Control.Exception |
| openBinaryFile | GHC.IO.Handle.FD, System.IO |
| openBinaryTempFile | System.IO |
| openBinaryTempFileWithDefaultPermissions | System.IO |
| openFile | GHC.IO.Handle.FD, System.IO |
| openFileBlocking | GHC.IO.Handle.FD |
| OpenKind | GHC.Exts |
| OpenPunctuation | Data.Char |
| openTempFile | System.IO |
| openTempFileWithDefaultPermissions | System.IO |
| OptArg | System.Console.GetOpt |
| OptDescr | System.Console.GetOpt |
| Option | System.Console.GetOpt |
| option | Text.ParserCombinators.ReadP |
| optional | |
| 1 (Function) | Text.ParserCombinators.ReadP |
| 2 (Function) | Control.Applicative |
| or | |
| 1 (Function) | GHC.OldList |
| 2 (Function) | Data.Foldable, Data.List, Prelude |
| or# | GHC.Exts |
| Ord | Data.Ord, Prelude |
| ord | Data.Char |
| ord# | GHC.Exts |
| Ordering | Data.Ord, Prelude |
| orElse | GHC.Conc.Sync, GHC.Conc |
| orI# | GHC.Exts |
| os | System.Info |
| OtherLetter | Data.Char |
| OtherNumber | Data.Char |
| OtherPunctuation | Data.Char |
| OtherSymbol | Data.Char |
| otherwise | Data.Bool, Prelude |
| outer | Data.Type.Equality |
| outputNL | GHC.IO.Handle, System.IO |
| OutputUnderflow | GHC.IO.Encoding.Types, GHC.IO.Encoding |
| Overflow | Control.Exception.Base, Control.Exception |