Preheat the oven to 400°F (200°C).
Pour the pizza sauce into a small saucepan and simmer over low heat until reduced to 1 cup, about 20 minutes; stir occasionally to prevent sticking.
Trim any excess fat from the chicken breasts and cut each breast crosswise into two equal pieces.
Place one chicken piece at a time inside a heavy plastic bag and gently pound with a meat mallet or heavy pan until thin but not torn; repeat with all pieces.
Mix the oregano and garlic powder in a small bowl, then sprinkle the mixture evenly on both sides of each chicken piece.
Heat the olive oil in a large nonstick frying pan over medium-high heat. Add chicken pieces (work in batches if needed) and brown 1–2 minutes per side—just until lightly browned but not cooked through.
Choose a glass casserole dish that fits the chicken in a single layer and arrange the browned chicken pieces in the dish.
Spread the reduced pizza sauce evenly over each piece of chicken, using the entire cup of sauce.
Top each sauced chicken piece with slices of mozzarella to mostly cover the surface, then place pepperoni slices over the cheese.
Bake uncovered for 25–30 minutes, until the cheese is melted and beginning to brown and the pepperoni edges are slightly crisp.
Remove from oven and serve hot.