Index - O
| odd | Base, Universum |
| on | Universum |
| OnDecodeError | String, Universum |
| One | Containers, Universum |
| one | Containers, Universum |
| OneItem | Containers, Universum |
| OnError | String, Universum |
| openFile | Lifted.File, Lifted, Universum |
| Option | |
| 1 (Data Constructor) | Universum |
| 2 (Type/Class) | Universum |
| optional | Universum |
| or | Containers, Universum |
| Ord | Universum |
| ord | Base, Universum |
| Ordering | Base, Universum |
| ordNub | Nub |
| orM | Monad, Universum |
| otherwise | Universum |
| over | Universum |
| overflowError | Base, Universum |