There is the AST defined in /parsing/parsetree.mli, which you have
undoubtedly discovered, and if you look at it carefully and search for the
occurences of different constructors in /parsing/parser.mly, you should be
able to figure where and how they are used.

Furthermore, you should try compiling sample programs with "ocamlc
-dparsetree" which will print the AST of the program.

Unfortunately, I am busy in these months, but you can send me some questions
and I can try to answer them (may take long time), as I have spent quite
some time inspecting the OCaml sources.
- Tom