| Object | GHC.Dotnet |
| OccName | Language.Haskell.TH.Syntax |
| October | System.Time |
| OddParity | System.Posix, System.Posix.Terminal |
| OneLineMode | Text.PrettyPrint, Text.PrettyPrint.HughesPJ |
| 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 |
| OpenPunctuation | Data.Char |
| Operator | Text.ParserCombinators.Parsec.Expr |
| OperatorTable | Text.ParserCombinators.Parsec.Expr |
| OptArg | System.Console.GetOpt |
| OptDescr | System.Console.GetOpt |
| Option | System.Console.GetOpt |
| Ord | Data.Ord, Prelude |
| Ordering | Data.Ord, Prelude |
| OtherLetter | Data.Char |
| OtherNumber | Data.Char |
| OtherPunctuation | Data.Char |
| OtherSymbol | Data.Char |
| OutputQueue | System.Posix, System.Posix.Terminal |
| Overflow | Control.Exception |
| occString | Language.Haskell.TH.Syntax |
| octDigit | Text.ParserCombinators.Parsec, Text.ParserCombinators.Parsec.Char |
| octal | Text.ParserCombinators.Parsec.Token |
| odd | Prelude |
| onNewLine | System.Console.Readline |
| onNewLineWithPrompt | System.Console.Readline |
| oneOf | Text.ParserCombinators.Parsec, Text.ParserCombinators.Parsec.Char |
| opLetter | Text.ParserCombinators.Parsec.Language, Text.ParserCombinators.Parsec.Token |
| opPrec | Language.Haskell.TH.Ppr |
| opStart | Text.ParserCombinators.Parsec.Language, Text.ParserCombinators.Parsec.Token |
| openBinaryFile | System.IO |
| openBinaryTempFile | System.IO |
| openDirStream | System.Posix, System.Posix.Directory |
| openEndedPipe | System.Posix.Signals, System.Posix, System.Posix.Signals.Exts |
| openFd | System.Posix, System.Posix.IO |
| openFile | System.IO |
| openTempFile | System.IO |
| operator | Text.ParserCombinators.Parsec.Token |
| option |
| 1 (Function) | Text.ParserCombinators.Parsec, Text.ParserCombinators.Parsec.Combinator |
| 2 (Function) | Text.ParserCombinators.ReadP |
| optional |
| 1 (Function) | Control.Applicative |
| 2 (Function) | Text.ParserCombinators.Parsec, Text.ParserCombinators.Parsec.Combinator |
| 3 (Function) | Text.ParserCombinators.ReadP |
| or |
| 1 (Function) | Data.Foldable |
| 2 (Function) | Data.List, Prelude |
| or# | GHC.Exts, GHC.Prim |
| orElse |
| 1 (Function) | Data.Generics, Data.Generics.Aliases |
| 2 (Function) | GHC.Conc, Control.Concurrent.STM, Control.Monad.STM |
| orInteger# | GHC.Exts, GHC.Prim |
| ord | Data.Char |
| ord# | GHC.Exts, GHC.Prim |
| 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, 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 |
