"skinny" greek pizza

Ingredients

• 1 Tbsp extra-virgin olive oil
• 4 (6-inch) whole wheat pita breads (100 calories each)
• 1 Tbsp red wine vinegar
• salt and pepper, to taste
• 1 tsp fresh oregano, chopped
• 1/2 small red onion, halved and sliced thin
• 12 pitted kalamata olives, halved
• 1/2 cup grape tomatoes, halved
• 1/2 cup hummus
• 1/2 part-skim mozzarella cheese, shredded
• 4 Tbsp (1 oz) crumbled feta cheese
• Tzatziki
• 1/2 cup nonfat plain Greek yogurt
• 3 Tbsp English cucumber, finely diced
• 1 clove garlic, finely minced
• 1 tsp white wine vinegar
• 1 tsp lemon juice
• salt and pepper, to taste