| ObjectiveC | Distribution.Simple.CCompiler | 
| ObjectiveCPlusPlus | Distribution.Simple.CCompiler | 
| objExtension | Distribution.Simple.BuildPaths | 
| oldExtensions | Distribution.PackageDescription | 
| OldPackageKey | Distribution.Package, Distribution.Make, Distribution.Simple | 
| 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 | 
| OptionBool | Distribution.TestSuite | 
| optionDefault | Distribution.TestSuite | 
| OptionDescr |  | 
| 1 (Type/Class) | Distribution.TestSuite | 
| 2 (Data Constructor) | Distribution.TestSuite | 
| optionDescr | Distribution.Simple.Command | 
| optionDescription | Distribution.TestSuite | 
| OptionEnum | Distribution.TestSuite | 
| OptionField |  | 
| 1 (Type/Class) | Distribution.Simple.Command | 
| 2 (Data Constructor) | Distribution.Simple.Command | 
| OptionFile | Distribution.TestSuite | 
| optionFileExtensions | Distribution.TestSuite | 
| optionFileIsDir | Distribution.TestSuite | 
| optionFileMustExist | Distribution.TestSuite | 
| optionName |  | 
| 1 (Function) | Distribution.TestSuite | 
| 2 (Function) | Distribution.Simple.Command | 
| OptionNumber | Distribution.TestSuite | 
| optionNumberBounds | Distribution.TestSuite | 
| optionNumberIsInt | Distribution.TestSuite | 
| optionNumJobs | Distribution.Simple.Setup | 
| OptionRngSeed | Distribution.TestSuite | 
| Options | Distribution.TestSuite | 
| options |  | 
| 1 (Function) | Distribution.TestSuite | 
| 2 (Function) | Distribution.PackageDescription | 
| OptionSet | Distribution.TestSuite | 
| OptionString | Distribution.TestSuite | 
| optionStringMultiline | Distribution.TestSuite | 
| OptionType | Distribution.TestSuite | 
| optionType | Distribution.TestSuite | 
| optionVerbosity | Distribution.Simple.Setup | 
| optKeepTempFiles | Distribution.Simple.Utils | 
| ordNub | Distribution.Simple.Utils | 
| ordNubRight | Distribution.Simple.Utils | 
| orEarlierVersion | Distribution.Version, Distribution.Simple | 
| OriginalModule |  | 
| 1 (Type/Class) | Distribution.InstalledPackageInfo | 
| 2 (Data Constructor) | Distribution.InstalledPackageInfo | 
| originalModuleName | Distribution.InstalledPackageInfo | 
| originalPackageId | Distribution.InstalledPackageInfo | 
| 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 | 
| otherExtensions | Distribution.PackageDescription | 
| otherLanguages | Distribution.PackageDescription | 
| OtherLicense | Distribution.License, Distribution.Make, Distribution.Simple | 
| otherModules | Distribution.PackageDescription | 
| OtherOS | Distribution.System | 
| OtherRepoType | Distribution.PackageDescription | 
| OverlappingInstances | Language.Haskell.Extension, Distribution.Simple | 
| OverloadedLists | Language.Haskell.Extension, Distribution.Simple | 
| OverloadedStrings | Language.Haskell.Extension, Distribution.Simple | 
| overNubList | Distribution.Utils.NubList | 
| overNubListR | Distribution.Utils.NubList |