Index - O
| odd | BasePrelude |
| on | BasePrelude |
| oneBits | BasePrelude |
| onException | BasePrelude |
| Op | |
| 1 (Type/Class) | BasePrelude |
| 2 (Data Constructor) | BasePrelude |
| openBinaryFile | BasePrelude |
| openBinaryTempFile | BasePrelude |
| openBinaryTempFileWithDefaultPermissions | BasePrelude |
| openFile | BasePrelude |
| OpenPunctuation | BasePrelude |
| openTempFile | BasePrelude |
| openTempFileWithDefaultPermissions | BasePrelude |
| optional | BasePrelude |
| or | BasePrelude |
| Ord | BasePrelude |
| ord | BasePrelude |
| Ord1 | BasePrelude |
| Ord2 | BasePrelude |
| Ordering | BasePrelude |
| OtherError | BasePrelude |
| OtherLetter | BasePrelude |
| OtherNumber | BasePrelude |
| OtherPunctuation | BasePrelude |
| OtherSymbol | BasePrelude |
| otherwise | BasePrelude |
| outputNL | BasePrelude |
| Overflow | BasePrelude |