Heat oil in a large nonstick skillet over medium-high heat. Add chicken and cook 6 minutes, stirring occasionally. Remove chicken.
Heat pan over medium-high heat. Add frozen pepper stir-fry and garlic; cook, stirring occasionally, 7 minutes or until softened. Stir in tomato, oregano, salt, chile pepper, and reserved chicken. Cook, stirring occasionally, 3 minutes or until chicken is done.
Heat tortillas according to package directions. Spoon chicken mixture evenly over each tortilla; sprinkle each with 1 tablespoon cheese. Fold bottom and sides of tortillas to center.