Preheat the oven to 400 degrees F (200 degrees C).
Place tortilla on a baking sheet. Brush with olive oil; sprinkle garlic powder, salt, and pepper on top.
Bake in the preheated oven until golden, 3 to 5 minutes. Remove from the oven and spread tomato sauce on top; arrange chicken breast, green bell pepper, and green onions over tomato sauce. Sprinkle with mozzarella cheese.
Bake in the preheated oven until cheese is melted, about 5 minutes.
Sprinkle oregano over cheese. Slice pizza into wedges using a pizza cutter.