| OOBInline | Network.Socket |
| Object | GHC.Dotnet |
| October | System.Time, Time |
| OddParity | System.Posix, System.Posix.Terminal |
| OneLineMode | Text.PrettyPrint, Text.PrettyPrint.HughesPJ, Language.Haskell.Pretty |
| OpenFileFlags |
| 1 (Type/Class) | System.Posix, System.Posix.IO |
| 2 (Data Constructor) | System.Posix, System.Posix.IO |
| OpenFileLimit | System.Posix, System.Posix.Unistd |
| OpenMode | System.Posix, System.Posix.IO |
| Operator | Text.ParserCombinators.Parsec.Expr |
| OperatorTable | Text.ParserCombinators.Parsec.Expr |
| OptArg | System.Console.GetOpt |
| OptDescr | System.Console.GetOpt |
| Option | System.Console.GetOpt |
| Ord | GHC.Base, Prelude |
| OrdALPHA | Debug.QuickCheck.Poly |
| OrdBETA | Debug.QuickCheck.Poly |
| OrdGAMMA | Debug.QuickCheck.Poly |
| Ordering | GHC.Base, Prelude |
| OtherError | GHC.IOBase |
| OutputQueue | System.Posix, System.Posix.Terminal |
| Overflow | Control.Exception, GHC.Exception, GHC.IOBase |
| octDigit | Text.ParserCombinators.Parsec, Text.ParserCombinators.Parsec.Char |
| octal | Text.ParserCombinators.Parsec.Token |
| odd | GHC.Real, Prelude |
| ok | Debug.QuickCheck |
| olist | Text.Html |
| olive | Text.Html |
| onNewLine | System.Console.Readline |
| onNewLineWithPrompt | System.Console.Readline |
| oneInt | GHC.Base |
| oneOf | Text.ParserCombinators.Parsec, Text.ParserCombinators.Parsec.Char |
| oneof | Debug.QuickCheck |
| onsideIndent | Language.Haskell.Pretty |
| opLetter | Text.ParserCombinators.Parsec.Language, Text.ParserCombinators.Parsec.Token |
| opPrec | Language.Haskell.THSyntax |
| opStart | Text.ParserCombinators.Parsec.Language, Text.ParserCombinators.Parsec.Token |
| openBinaryFile | GHC.Handle, System.IO |
| openDirStream | System.Posix, System.Posix.Directory |
| openEndedPipe | System.Posix.Signals, System.Posix |
| openFd |
| 1 (Function) | GHC.Handle |
| 2 (Function) | System.Posix, System.Posix.IO |
| openFile | GHC.Handle, System.IO, IO |
| operator | Text.ParserCombinators.Parsec.Token |
| option |
| 1 (Function) | Text.Html |
| 2 (Function) | Text.ParserCombinators.Parsec, Text.ParserCombinators.Parsec.Combinator |
| optional | Text.ParserCombinators.Parsec, Text.ParserCombinators.Parsec.Combinator |
| or | Data.List, GHC.List, Prelude, List |
| orElse | Data.Generics, Data.Generics.Aliases |
| ord | Data.Char, GHC.Base, Char |
| ordList | Text.Html |
| os | System.Info |
| otherExecuteMode | System.Posix, System.Posix.Files |
| otherModes | System.Posix, System.Posix.Files |
| otherReadMode | System.Posix, System.Posix.Files |
| otherWriteMode | System.Posix, System.Posix.Files |
| otherwise | Data.Bool, GHC.Base, Prelude |
| outdegree | Data.Graph |
| outputSpeed | System.Posix, System.Posix.Terminal |
| ownerExecuteMode | System.Posix, System.Posix.Files |
| ownerModes | System.Posix, System.Posix.Files |
| ownerReadMode | System.Posix, System.Posix.Files |
| ownerWriteMode | System.Posix, System.Posix.Files |
