| October | System.Time |
| OneLineMode | Text.PrettyPrint Text.PrettyPrint.HughesPJ |
| OptArg | System.Console.GetOpt |
| Option | System.Console.GetOpt |
| 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 |
| octal | Text.ParserCombinators.Parsec.Token |
| odd | GHC.Real Prelude |
| ok | Debug.QuickCheck |
| olist | Text.Html |
| olive | Text.Html |
| oneInt | GHC.Base |
| 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 | Text.Html 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 |
