OOAD helps us in breaking down a problem domain into, so to speak, a set of classes that collaborate to provide a solution.
1. If you talk to XP (eXtreme Prgoramming)folks they'll scoff at the idea of having any abstractions other than the code itself. To a large extent they may be right because beyond a certian level (of abstraction) models become useless, and the code is both the detailed design & the documentation. You can go the XP sites for more information.
2. If instead you take a more conventional approach, a Visio chart or even an Activity diagram (that UML provides) will help you work at that level.