Crumble oregano into a small dry skillet. Toast over low heat for a minute or two, stirring often to avoid burning. Add oil, vinegar, and minced garlic, salt and freshly ground black pepper; heat about one minute. Transfer to a large bowl and whisk until combined.
Drain the beans and place in a large saucepan. Stir in 1/3 cup of the dressing and let beans marinate for about 30 minutes. Add onion to the remaining dressing and marinate this for 30 minutes as well. Meanwhile, peel, core and cut pears into half to one-inch pieces. After beans are marinated, add the spinach and feta cheese to the beans and toss gently with the onion dressing that was marinating. Serve topped with olives. Yields 6 servings.