How is Ontological Design used? What can it be applied to?
This is the real beauty of Ontological Design. It is not so much a design methodology as a design philosophy, and one that can be used to validate/test any system design and implementation.
..and any type of system - not just computer based.
The design and system implementation can be carried out using whatever approach is most familiar. Although it MUST be iterative/incremental. Each design/implementation increment can then be tested using the principles of Ontological Design.
At each level in the system do the individual components present interfaces that are "Ready To Hand"?
At each level, how are errors or exceptional events handled? They should not introduce unexpected behaviour for which the user is unprepared - in terms of the expected understood concepts.
At each level address the issue of "blindness" - What is this component for? What conceptual abstractions have been made that might not be correct, or could be improved?
...and perhaps most importantly.
What is the system and the design.
What is the system behaviour?
When implementing, fixing, modifying.... is the resulting behaviour truly understandable in terms of the concepts introduced?
At every level?
No matter how hard you try, it is not possible to be sure that a user develops the intended conceptual representation.
Therefore, it should be understood that the process of learning is one which necessitates elements of breakdown due to the blindness of inappropriate internal representations.
Sensitivity to the multi-level nature of breakdown and blindness should be key aspect of system analysis using these principals. And should inform design decisions along the way.