1. Cut off and discard head of octopus, then cut tentacles into 1-inch pieces. Generously cover octopus with water in a heavy medium pot and gently simmer, uncovered, until tender, 45 minutes to 1 hour.
2. Drain octopus in a colander and cool to room temperature, then transfer to a bowl. Stir in remaining ingredients, 1/2 teaspoon pepper, and additional sea salt to taste.
3. Let stand 30 minutes for flavors to develop.
4. Cooks' note: Octopus salad, without parsley, can be made 1 day ahead and chilled, covered. Stir in parsley just before serving.