Fennel provides a faintly sweet counterpoint to the bitter flavor of radicchio.
1. In the container of a blender or processor, combine 5 of the olives, the oil, lemon juice, anchovies, salt, and cayenne. Whirl until smooth and blend in the oregano.
2. Trim the fennel, cutting off the root and leaf stems to form a bulb. Slice the bulb lengthwise into 8 wedges, and then separate each wedge into layers.
3. Separate the radicchio leaves and rinse them. Drain on absorbent paper and pat dry. Divide among 4 salad plates, interspersing the radicchio leaves with fennel. Coarsely chop the remaining 5 olives and scatter them on top of each salad. Pour on the dressing and serve at once.