Preheat oven to 350°.
Combine onion powder, paprika, chili powder, and 1/2 teaspoon salt; sprinkle over chicken.
Heat oil in a large nonstick skillet over medium-high heat. Add the chicken, and sauté 2 minutes on each side or until browned. Wrap handle of skillet in foil; bake at 350° for 10 minutes or until done. Remove from pan; refrigerate until chilled. Chop into bite-sized pieces.
Combine 1/4 teaspoon salt, green grapes, and next 6 ingredients (green grapes through orange juice) in a large bowl. Add chopped chicken, and toss to coat. Sprinkle salad with walnuts.