Marinate chicken: Combine chicken with olive oil, lemon juice, garlic, oregano, salt, and pepper. Let sit 15 minutes.
Make tzatziki: Grate cucumber, squeeze out excess water, mix with yogurt, garlic, lemon, dill, and salt. Chill.
Cook chicken: Heat skillet over medium-high, cook chicken 3-4 minutes per side until golden. Rest 5 minutes, then slice.
Cook pasta (if using): Boil orzo according to package, drain well.
Assemble pasta: Toss hot pasta with sliced chicken, tomatoes, cucumber, onion, olives, feta, and 1/2 cup tzatziki.
Assemble bowls: Layer grains, chicken, tzatziki, vegetables, feta, and olives.
Serve with extra tzatziki and lemon wedges.