Heat 1 teaspoon oil in a large nonstick skillet over medium-high heat.
Add onion and peppers; cook 3 minutes or until lightly browned (do not stir).
Remove onion and peppers from pan; place in a large bowl.
Heat 1/2 teaspoon oil in skillet.
Add zucchini and squash, and cook 3 minutes or until lightly browned (do not stir).
Remove zucchini and squash from pan, and place in bowl.
Heat 1/2 teaspoon oil in skillet.
Add mushrooms and garlic, and cook 3 minutes or until lightly browned (do not stir). Return onion, peppers, zucchini, and squash to skillet. Stir in rice, oregano, pepper, and salt, and cook just until heated.