Index - O
| odd | BasePrelude |
| on | BasePrelude |
| onException | BasePrelude |
| OpenPunctuation | BasePrelude |
| optional | BasePrelude |
| or | BasePrelude |
| Ord | BasePrelude |
| ord | BasePrelude |
| Ordering | BasePrelude |
| orElse | BasePrelude |
| OtherError | BasePrelude |
| OtherLetter | BasePrelude |
| OtherNumber | BasePrelude |
| OtherPunctuation | BasePrelude |
| OtherSymbol | BasePrelude |
| otherwise | BasePrelude |
| Overflow | BasePrelude |