You may be confused by the fact that the type system will notice the equivalence of two types if the implementations of both are public eg

    module M = 
    struct
       type newtons = int
       let inc some_newtons = some_newtons + 1
    end :
    sig
       type newtons = int
       val inc : newtons -> newtons
    end

    M.inc 1 (* this works *)

    module M = 
    struct
       type newtons = int
       let inc some_newtons = some_newtons + 1
       let in_newtons x = x 
    end :
    sig 
       type newtons
       val inc : newtons -> newtons
       val in_newtons : int -> newtons
    end

    M.inc 1 (* type error *)
    M.inc (in_newtons 1) (* this works *)

The use of hidden types in OCaml gives far better control over encapsulation and implementation hiding than any over language I've used.