| OneLineMode | Language.Haskell.Exts.Pretty, Language.Haskell.Exts | 
| onsideIndent | Language.Haskell.Exts.Pretty, Language.Haskell.Exts | 
| Op | Language.Haskell.Exts.Syntax, Language.Haskell.Exts | 
| op | Language.Haskell.Exts.Build, Language.Haskell.Exts | 
| OpenArrowBracket | Language.Haskell.Exts.Lexer, Language.Haskell.Exts | 
| OPTIONS | Language.Haskell.Exts.Lexer, Language.Haskell.Exts | 
| OptionsPragma | Language.Haskell.Exts.Syntax, Language.Haskell.Exts | 
| OrFormula | Language.Haskell.Exts.Syntax, Language.Haskell.Exts | 
| OVERLAP | Language.Haskell.Exts.Lexer, Language.Haskell.Exts | 
| Overlap |   | 
| 1 (Type/Class) | Language.Haskell.Exts.Syntax, Language.Haskell.Exts | 
| 2 (Data Constructor) | Language.Haskell.Exts.Syntax, Language.Haskell.Exts | 
| OVERLAPPABLE | Language.Haskell.Exts.Lexer, Language.Haskell.Exts | 
| Overlappable | Language.Haskell.Exts.Syntax, Language.Haskell.Exts | 
| OVERLAPPING | Language.Haskell.Exts.Lexer, Language.Haskell.Exts | 
| Overlapping | Language.Haskell.Exts.Syntax, Language.Haskell.Exts | 
| OverlappingInstances | Language.Haskell.Exts.Extension, Language.Haskell.Exts | 
| OVERLAPS | Language.Haskell.Exts.Lexer, Language.Haskell.Exts | 
| Overlaps | Language.Haskell.Exts.Syntax, Language.Haskell.Exts | 
| OverloadedLabel | Language.Haskell.Exts.Syntax, Language.Haskell.Exts | 
| OverloadedLabels | Language.Haskell.Exts.Extension, Language.Haskell.Exts | 
| OverloadedStrings | Language.Haskell.Exts.Extension, Language.Haskell.Exts |