| octal | Numeric.Lens |
| onceUpon | Data.Data.Lens, Control.Lens.Extras |
| onceUpon' | Data.Data.Lens, Control.Lens.Extras |
| One | Control.Lens.Internal.Level, Control.Lens.Internal |
| only | Control.Lens.Prism, Control.Lens, Control.Lens.Combinators |
| op | Control.Lens.Wrapped, Control.Lens, Control.Lens.Combinators |
| Optic | Control.Lens.Type, Control.Lens, Control.Lens.Combinators |
| Optic' | Control.Lens.Type, Control.Lens, Control.Lens.Combinators |
| Optical | Control.Lens.Type, Control.Lens, Control.Lens.Combinators |
| Optical' | Control.Lens.Type, Control.Lens, Control.Lens.Combinators |
| OpticP | Control.Lens.Profunctor |
| ordinalNub | Control.Lens.Internal.List |
| ordinals | |
| 1 (Function) | Data.Vector.Generic.Lens |
| 2 (Function) | Data.Vector.Lens |
| orOf | Control.Lens.Fold, Control.Lens, Control.Lens.Combinators |
| outside | Control.Lens.Prism, Control.Lens, Control.Lens.Combinators |
| Over | Control.Lens.Type, Control.Lens, Control.Lens.Combinators |
| over | Control.Lens.Setter, Control.Lens, Control.Lens.Combinators |
| Over' | Control.Lens.Type, Control.Lens, Control.Lens.Combinators |
| overA | Control.Lens.Lens, Control.Lens, Control.Lens.Combinators |
| overEquality | Control.Lens.Equality, Control.Lens, Control.Lens.Combinators |
| Overflow_ | Control.Exception.Lens |