Sorry, I am not very fluent in OCaml. Also, I didn't find word "message" in the link above.

I am experiencing limitations of pure objects and message passing almost right now. In my spare time I am currently developing dynamic data flow CPU which is based on immutable "objects" and message sending. It is quite unusual style of programming, very unlike anything else, including Smalltalk, Erlang and everything.

It's like Haskell, with a slightly heavier syntax. And it's eager. And you can unsafePerformIO all over the place (but avoid doing so even in Ocaml).

    (* Ocaml *)    --Haskell
    let foo = 42   foo = 42    -- top level declaration
    type bar =     data bar =  -- type declaration

Also, records in Ocaml are a separate kind of type, more like C structs:

    type bar = { x:int;     (* defination *)
                 y:float;
               }
    bar.x                   (* "member" acces *)

"Message passing" and "Method calling" are strictly equivalent, at least when everything is synchronous.