> This works, of course:
> 
>   class parent = object (self : 'self)
>     val mutable children : 'self list = []
>     method add_fresh_object = children <- {<>} :: children
>   end
> 
> But this is not the intended result: now we have added a *copy* of
> the current object, not of its "fresh" state, i.e. the state it was in
> immediately after creation.

You could use an initializer to remember the "fresh" state in an instance
variable (using {< >}), and use a copy of that instance variable
in the method add_fresh_object. 

    class parent = object (self : 'self)
      val mutable fresh = None
      val mutable children : 'self list = []
      method add_fresh_object =
        let Some x = fresh in children <- Oo.copy x :: children
      initializer  fresh <- Some {< >}
    end;;

Anyway, you have apparently found your own solution.

Best regards,

        Didier