For the dressing: In a large bowl, whisk together the olive oil, vinegar, salt and oregano.
For the salad: To the dressing, add the cooked rice, prosciutto, cheese, peppers, peas, olives, salt, fennel and tuna and basil if using. Toss well to coat in the dressing. Allow to sit at room temperature for 15 minutes to let the flavors marry. Top with the hard-boiled eggs if using and serve.