Preheat oven to 425°F.
Heat oil in a skillet over medium heat. Add onion and bell pepper, and sauté 3 minutes. Stir in chicken and barbecue sauce, and remove from heat.
Arrange pita bread on a baking sheet, and top evenly chicken mixture, cheese, and pineapple.
Bake 10 to 12 minutes or until cheese melts and pitas are crisp.