| Arg | Parsing of command line arguments. |
| Array | Array operations. |
| ArrayLabels | Array operations. |
| Atomic | Atomic references. |
| Bigarray | Large, multi-dimensional, numerical arrays. |
| Bool | Boolean values. |
| Buffer | Extensible buffers. |
| Bytes | Byte sequence operations. |
| BytesLabels | Byte sequence operations. |
| Callback | Registering OCaml values with the C runtime. |
| Char | Character operations. |
| Complex | Complex numbers. |
| Digest | Message digest. |
| Domain | |
| Dynarray | Dynamic arrays. |
| Dynlink | Dynamic loading of .cmo, .cma and .cmxs files. |
| Dynlink_types | Types shared amongst the various parts of the dynlink code. |
| Effect | |
| Either | Either type. |
| Ephemeron | Ephemerons and weak hash tables. |
| Event | First-class synchronous communication. |
| Filename | Operations on file names. |
| Float | Floating-point arithmetic. |
| Fun | Function manipulation. |
| Gc | Memory management control and statistics; finalised values. |
| Hashtbl | Hash tables and hash functions. |
| In_channel | Input channels. |
| Int | Integer values. |
| Int32 | 32-bit integers. |
| Int64 | 64-bit integers. |
| Lazy | Deferred computations. |
| Lexing | The run-time library for lexers generated by |
| List | List operations. |
| ListLabels | List operations. |
| Map | Association tables over ordered types. |
| Marshal | Marshaling of data structures. |
| Mutex | Locks for mutual exclusion. |
| Nativeint | Processor-native integers. |
| Obj | Operations on internal representations of values. |
| Oo | Operations on objects |
| Option | Option values. |
| Out_channel | Output channels. |
| Parsing | The run-time library for parsers generated by |
| Printexc | Facilities for printing exceptions and inspecting current call stack. |
| Printf | Formatted output functions. |
| Queue | First-in first-out queues. |
| Random | Pseudo-random number generators (PRNG). |
| Result | Result values. |
| Scanf | Formatted input functions. |
| Semaphore | Semaphores |
| Seq | Sequences. |
| Set | Sets over ordered types. |
| Stack | Last-in first-out stacks. |
| StdLabels | Standard labeled libraries. |
| Std_exit | |
| Str | Regular expressions and high-level string processing |
| String | Strings. |
| StringLabels | Strings. |
| Sys | System interface. |
| Type | Type introspection. |
| Uchar | Unicode characters. |
| Unit | Unit values. |
| Unix | Interface to the Unix system. |
| Weak | Arrays of weak pointers and hash sets of weak pointers. |