Chopped Veggie Salad with Watermelon and Feta Cheese

1 pound Campari or plum tomatoes, diced, drained 1 1/2 cups diced seeded watermelon 1 large green bell pepper, seeded, cut into 1/3-inch cubes 1/2 large English hothouse cucumber, seeded, cut into 1/3-inch cubes 1/2 cup very thinly sliced radishes 3 tablespoons olive oil, divided 10 ounces feta cheese, broken into small cubes (about 2 1/2 cups), divided 2 green onions, chopped, divided 1/4 cup thinly sliced fresh mint leaves, divided 1/2 cup plain Greek-style yogurt* 1 teaspoon dried oregano *A thick yogurt; sold at some supermarkets and at specialty foods stores (such as Trader Joe's and Whole Foods Markets) and Greek markets. If unavailable, place regular yogurt in cheesecloth-lined strainer set over large bowl. Cover and chill overnight to drain.