Index - O
| odd | RIO |
| on | RIO |
| onException | RIO |
| OpenPunctuation | RIO.Char |
| optional | RIO |
| or | |
| 1 (Function) | RIO.List, RIO |
| 2 (Function) | RIO.Vector.Unboxed |
| 3 (Function) | RIO.Vector.Storable |
| 4 (Function) | RIO.Vector.Boxed |
| 5 (Function) | RIO.Vector |
| Ord | RIO |
| ord | RIO.Char |
| Ordering | RIO |
| OtherLetter | RIO.Char |
| OtherNumber | RIO.Char |
| OtherPunctuation | RIO.Char |
| OtherSymbol | RIO.Char |
| otherwise | RIO |
| over | RIO |