| objExtension | Distribution.Simple.BuildPaths |
| OpenBSD | Distribution.System |
| OptArg | Distribution.Simple.Command |
| optArg | Distribution.Simple.Command |
| optArg' | Distribution.Simple.Command |
| OptDescr | Distribution.Simple.Command |
| OptFlags | Distribution.Simple.Command |
| OptimisationLevel | Distribution.Simple.Compiler, Distribution.Simple |
| option | |
| 1 (Function) | Distribution.Compat.ReadP |
| 2 (Function) | Distribution.Simple.Command |
| optional | Distribution.Compat.ReadP |
| optionDescr | Distribution.Simple.Command |
| OptionField | |
| 1 (Type/Class) | Distribution.Simple.Command |
| 2 (Data Constructor) | Distribution.Simple.Command |
| optionName | Distribution.Simple.Command |
| options | Distribution.PackageDescription |
| optionVerbosity | Distribution.Simple.Setup |
| orEarlierVersion | Distribution.Version, Distribution.Simple |
| orLaterVersion | Distribution.Version, Distribution.Simple |
| OS | |
| 1 (Type/Class) | Distribution.System |
| 2 (Data Constructor) | Distribution.PackageDescription |
| OSVar | Distribution.Simple.InstallDirs, Distribution.Simple.LocalBuildInfo |
| OSX | Distribution.System |
| OtherArch | Distribution.System |
| OtherCompiler | Distribution.Compiler, Distribution.Simple.Compiler, Distribution.Simple |
| OtherLicense | Distribution.License, Distribution.Make, Distribution.Simple |
| otherModules | Distribution.PackageDescription |
| OtherOS | Distribution.System |
| OtherRepoType | Distribution.PackageDescription |
| OverlappingInstances | Language.Haskell.Extension, Distribution.Simple |
| OverloadedStrings | Language.Haskell.Extension, Distribution.Simple |