1 lb. beef round tip steaks, cut 1/8- to 1/4-inch thick
2 cloves garlic, minced
1 tsp. dried oregano
3 tsp. olive oil
1 medium onion, halved, thinly sliced
1/4 tsp. salt
1/8 tsp. pepper
4 pita breads, cut in half, warmed
2 small tomatoes, thinly sliced
1/2 cup prepared cucumber ranch dressing