| OS |
| 1 (Data Constructor) | Distribution.Configuration |
| 2 (Type/Class) | Distribution.System |
| OSX | Distribution.System |
| Object | GHC.Dotnet |
| OccName | Language.Haskell.TH.Syntax |
| October | Time, System.Time |
| OddParity | System.Posix.Terminal, System.Posix |
| OneLineMode | Text.PrettyPrint.HughesPJ, Text.PrettyPrint |
| OpenFileFlags |
| 1 (Type/Class) | System.Posix.IO, System.Posix |
| 2 (Data Constructor) | System.Posix.IO, System.Posix |
| OpenFileLimit | System.Posix.Unistd, System.Posix |
| OpenMode | System.Posix.IO, System.Posix |
| OpenPunctuation | Data.Char |
| OpenSemFlags |
| 1 (Type/Class) | System.Posix.Semaphore, System.Posix |
| 2 (Data Constructor) | System.Posix.Semaphore, System.Posix |
| OptArg | System.Console.GetOpt |
| OptDescr | System.Console.GetOpt |
| Option | System.Console.GetOpt |
| Ord | Data.Ord, Prelude |
| Ordering | Data.Ord, Prelude |
| Other | Distribution.System |
| OtherCompiler | Distribution.Compiler, Distribution.Simple.Compiler, Distribution.Simple.Setup, Distribution.Setup, Distribution.Simple |
| OtherLetter | Data.Char |
| OtherLicense | Distribution.License, Distribution.Make, Distribution.Simple |
| OtherNumber | Data.Char |
| OtherPunctuation | Data.Char |
| OtherSymbol | Data.Char |
| OutputQueue | System.Posix.Terminal, System.Posix |
| Overflow | Control.Exception |
| OverlappingInstances | Language.Haskell.Extension, Distribution.Extension, Distribution.Simple |
| OverloadedStrings | Language.Haskell.Extension, Distribution.Extension, Distribution.Simple |
| objExtension | Distribution.Simple.Utils |
| occString | Language.Haskell.TH.Syntax |
| odd | Prelude |
| on | Data.Function |
| onNewLine | System.Console.Readline |
| onNewLineWithPrompt | System.Console.Readline |
| opPrec | Language.Haskell.TH.Ppr |
| openBinaryFile | System.IO |
| openBinaryTempFile | System.IO |
| openDirStream | System.Posix.Directory, System.Posix |
| openEndedPipe | System.Posix.Signals, System.Posix.Signals.Exts, System.Posix |
| openFd | System.Posix.IO, System.Posix |
| openFile | System.IO, IO |
| openPseudoTerminal | System.Posix.Terminal, System.Posix |
| openTempFile | System.IO |
| option | Distribution.Compat.ReadP, Text.ParserCombinators.ReadP |
| optional |
| 1 (Function) | Control.Applicative |
| 2 (Function) | Distribution.Compat.ReadP, Text.ParserCombinators.ReadP |
| options | Distribution.PackageDescription |
| or |
| 1 (Function) | Data.Foldable |
| 2 (Function) | Data.List, Prelude, List |
| or# | GHC.Prim, GHC.Exts |
| orEarlierVersion | Distribution.Version, Distribution.Simple |
| orElse |
| 1 (Function) | Data.Generics.Aliases, Data.Generics |
| 2 (Function) | GHC.Conc |
| orInteger# | GHC.Prim, GHC.Exts |
| orLaterVersion | Distribution.Version, Distribution.Simple |
| ord | Data.Char, Char |
| ord# | GHC.Prim, GHC.Exts |
| os |
| 1 (Function) | Distribution.System |
| 2 (Function) | System.Info |
| otherExecuteMode | System.Posix.Files, System.Posix |
| otherModes | System.Posix.Files, System.Posix |
| otherModules | Distribution.PackageDescription |
| otherReadMode | System.Posix.Files, System.Posix |
| otherWriteMode | System.Posix.Files, System.Posix |
| otherwise | Data.Bool, Prelude |
| outdegree | Data.Graph |
| outputSpeed | System.Posix.Terminal, System.Posix |
| ownerExecuteMode | System.Posix.Files, System.Posix |
| ownerModes | System.Posix.Files, System.Posix |
| ownerReadMode | System.Posix.Files, System.Posix |
| ownerWriteMode | System.Posix.Files, System.Posix |
