The Objective Caml compiler has a special option "-pp", allowing to call any preprocessor. If Camlp4 is used, the parsing is done (only once) by Camlp4, and the Objective Caml compiler resumes the rest of the compilation (typing, code generation).
Camlp4 can parse normal Ocaml concrete syntax or any other user-definable syntax. As an example, an alternative syntax is provided, named revised, because it tries to fix some problems of the normal syntax.
Camlp4 can pretty print the normal Ocaml concrete syntax or the revised one. It is therefore always possible to have a version of your sources compilable by the compiler Objective Caml without preprocessing.
Documentation (not yet up to date with the released version):
Daniel de Rauglaudre