| October | System.Time |
| OneLineMode | Text.PrettyPrint, Text.PrettyPrint.HughesPJ |
| 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 |
| Overflow | Control.Exception, GHC.Exception, GHC.IOBase |
| o_APPEND | GHC.Posix |
| o_BINARY | GHC.Posix |
| o_CREAT | GHC.Posix |
| o_EXCL | GHC.Posix |
| o_NOCTTY | GHC.Posix |
| o_NONBLOCK | GHC.Posix |
| o_RDONLY | GHC.Posix |
| o_RDWR | GHC.Posix |
| o_TRUNC | GHC.Posix |
| o_WRONLY | GHC.Posix |
| 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 |
| oneInt | GHC.Base |
| oneOf | Text.ParserCombinators.Parsec, Text.ParserCombinators.Parsec.Char |
| oneof | Debug.QuickCheck |
| opLetter | Text.ParserCombinators.Parsec.Language, Text.ParserCombinators.Parsec.Token |
| opStart | Text.ParserCombinators.Parsec.Language, Text.ParserCombinators.Parsec.Token |
| openFd | GHC.Handle |
| openFile | GHC.Handle, System.IO |
| openFileEx | GHC.Handle |
| 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 |
| ord | Data.Char, GHC.Base |
| ordList | Text.Html |
| os | System.Info |
| otherwise | Data.Bool, GHC.Base, Prelude |
