OCamlPro has published cheat sheets (one or two-page summaries) on OCaml:
The OCaml Language (PDF, June 2011)
General overview of the OCaml language: basic data types, basic concepts, functions, modules, etc.
OCaml Standard Tools (PDF, June 2011)
Overview of OCaml compilers and their options, tools for lexing and parsing, Makefile rules, etc.
OCaml Standard Library (PDF, June 2011)
Overview of the standard library's most common modules.
OCaml Emacs Mode (Tuareg) (PDF, June 2011)
Overview of the Emacs Tuareg mode keyboard shortcuts.