| OBool | Language.Sifflet.Expr |
| OChar | Language.Sifflet.Expr |
| offsetTextBoxCenters | Data.Sifflet.TreeLayout |
| on | Graphics.UI.Sifflet.LittleGtk, Graphics.UI.Sifflet |
| onDestroy | Graphics.UI.Sifflet.LittleGtk, Graphics.UI.Sifflet |
| OnEntryActivate | Graphics.UI.Sifflet.Callback, Graphics.UI.Sifflet |
| OnLayoutButtonPress | Graphics.UI.Sifflet.Callback, Graphics.UI.Sifflet |
| OnLayoutButtonRelease | Graphics.UI.Sifflet.Callback, Graphics.UI.Sifflet |
| OnLayoutExpose | Graphics.UI.Sifflet.Callback, Graphics.UI.Sifflet |
| OnLayoutMouseMove | Graphics.UI.Sifflet.Callback, Graphics.UI.Sifflet |
| OnMenuItemActivateLeaf | Graphics.UI.Sifflet.Callback, Graphics.UI.Sifflet |
| onSizeRequest | Graphics.UI.Sifflet.LittleGtk, Graphics.UI.Sifflet |
| OnWindowConfigure | Graphics.UI.Sifflet.Callback, Graphics.UI.Sifflet |
| OnWindowDestroy | Graphics.UI.Sifflet.Callback, Graphics.UI.Sifflet |
| opAssoc | Language.Sifflet.Expr |
| openFilePath | Graphics.UI.Sifflet.Window, Graphics.UI.Sifflet |
| openNode | Graphics.UI.Sifflet.Workspace, Graphics.UI.Sifflet |
| Operator | |
| 1 (Type/Class) | Language.Sifflet.Expr |
| 2 (Data Constructor) | Language.Sifflet.Expr |
| OperatorGrouping | Language.Sifflet.Expr |
| operatorTable | |
| 1 (Function) | Language.Sifflet.Export.Haskell |
| 2 (Function) | Language.Sifflet.Export.Python |
| opGrouping | Language.Sifflet.Expr |
| opName | Language.Sifflet.Expr |
| opPrec | Language.Sifflet.Expr |
| optionsHardMaxLineWidth | Language.Sifflet.Export.ToHaskell |
| optionsSoftMaxLineWidth | Language.Sifflet.Export.ToHaskell |
| orderedChildren | Data.Sifflet.TreeGraph |
| orderedTreeToGraph | Data.Sifflet.TreeGraph |
| OStr | Language.Sifflet.Expr |