I have a very simple rule of thumb when it comes to OO: OO belongs in modules.

Modules are an intermediate level, where I build abstractions that will probably be re-used by several programs. There it makes sense, if only in order not to pollute the main namespace, to have a clean, opaque, interface and to wrap data structures and methods.

On the other and, at application level, I have seldom found the need to use OO design. On the contrary I often find that it would get in the way, as this is the level where most of the time abstractions hit the reality wall (I could have used an other expression involving stuff hitting revolving blades) and where good ole top down procedural design usually works well.

Of course this is not an absolute rule but I have found that it worked pretty well for me, with the added advantage of letting me be abstract, clean and happy with my code in OO modules while I know that application-level code is always a little messier and I have to live with it.