| O | GHC.Base | 
| objectName | GHC.ExecutionStack.Internal, GHC.ExecutionStack | 
| odd | GHC.Real, Prelude | 
| oldGenFactor | GHC.RTS.Flags | 
| on | Data.Function | 
| OneLineGCStats | GHC.RTS.Flags | 
| OneShot | GHC.Event | 
| oneShot | GHC.Base, GHC.Exts | 
| onException |  | 
| 1 (Function) | GHC.IO | 
| 2 (Function) | Control.Exception, Control.Exception.Base | 
| Op |  | 
| 1 (Type/Class) | Data.Functor.Contravariant | 
| 2 (Data Constructor) | Data.Functor.Contravariant | 
| Opaque | GHC.Base | 
| openBinaryFile |  | 
| 1 (Function) | GHC.IO.Handle.FD | 
| 2 (Function) | System.IO, GHC.IO.StdHandles | 
| openBinaryTempFile | System.IO | 
| openBinaryTempFileWithDefaultPermissions | System.IO | 
| openFile |  | 
| 1 (Function) | GHC.IO.FD | 
| 2 (Function) | GHC.IO.Handle.FD | 
| 3 (Function) | System.IO, GHC.IO.StdHandles | 
| openFileBlocking |  | 
| 1 (Function) | GHC.IO.Handle.FD | 
| 2 (Function) | GHC.IO.StdHandles | 
| OpenPunctuation | GHC.Unicode, Data.Char | 
| openTempFile | System.IO | 
| openTempFileWithDefaultPermissions | System.IO | 
| openTextEncoding | GHC.IO.Handle.Internals | 
| OptArg | System.Console.GetOpt | 
| OptDescr | System.Console.GetOpt | 
| Option |  | 
| 1 (Data Constructor) | System.Console.GetOpt | 
| 2 (Type/Class) | Data.Semigroup | 
| 3 (Data Constructor) | Data.Semigroup | 
| option |  | 
| 1 (Function) | Text.ParserCombinators.ReadP | 
| 2 (Function) | Data.Semigroup | 
| optional |  | 
| 1 (Function) | Text.ParserCombinators.ReadP | 
| 2 (Function) | Control.Applicative | 
| or |  | 
| 1 (Function) | GHC.List, GHC.OldList | 
| 2 (Function) | Data.List, Data.Foldable, Prelude | 
| or# | GHC.Base, GHC.Exts | 
| Ord | GHC.Base, Data.Ord, Prelude | 
| ord | GHC.Base, Data.Char | 
| ord# | GHC.Base, GHC.Exts | 
| Ord1 | Data.Functor.Classes | 
| Ord2 | Data.Functor.Classes | 
| Ordering | GHC.Base, Data.Ord, Prelude | 
| orElse | GHC.Conc.Sync, GHC.Conc | 
| orI# | GHC.Base, GHC.Exts | 
| orInteger | GHC.Integer | 
| orNatural | GHC.Natural | 
| os | System.Info | 
| OtherError | GHC.IO.Exception | 
| OtherLetter | GHC.Unicode, Data.Char | 
| OtherNumber | GHC.Unicode, Data.Char | 
| OtherPunctuation | GHC.Unicode, Data.Char | 
| OtherSymbol | GHC.Unicode, Data.Char | 
| otherwise | GHC.Base, Data.Bool, Prelude | 
| outer | Data.Type.Equality | 
| outputNL | System.IO, GHC.IO.Handle.Types, GHC.IO.Handle | 
| OutputUnderflow | GHC.IO.Encoding, GHC.IO.Encoding.Types | 
| Overflow | GHC.Exception, Control.Exception, Control.Exception.Base, GHC.Exception.Type | 
| overflowError | GHC.Real | 
| overflowException | GHC.Exception, GHC.Exception.Type | 
| o_APPEND | System.Posix.Internals | 
| o_BINARY | System.Posix.Internals | 
| o_CREAT | System.Posix.Internals | 
| o_EXCL | System.Posix.Internals | 
| o_NOCTTY | System.Posix.Internals | 
| o_NONBLOCK | System.Posix.Internals | 
| o_RDONLY | System.Posix.Internals | 
| o_RDWR | System.Posix.Internals | 
| o_TRUNC | System.Posix.Internals | 
| o_WRONLY | System.Posix.Internals |