Place pita rounds on a large baking sheet. Spread each round with one-fourth of the hummus, leaving a 1-inch border. Sprinkle each with one-fourth of the tomatoes, artichoke hearts, feta, and mozzarella. If desired, drizzle with oil and sprinkle with sesame seed.
Bake in a 450 degree F oven for 8 to 10 minutes or until cheese has melted and edges are lightly browned. If desired, top with olives and oregano. Cut into quarters. Makes 16 appetizer servings.