Preheat oven to 375°F and grease a 9×13-inch baking dish.
Add cherry tomatoes, sun-dried tomatoes, spinach, red onion, olives, orzo, 1/2 teaspoon salt, and 1/2 teaspoon oregano to the dish. Stir to combine.
In a separate bowl, season chicken with remaining salt and oregano.
Add seasoned chicken and chicken broth to the casserole dish and stir gently.
Make a small well in the center and place the feta block in the middle.
Cover tightly and bake for 30 minutes.
Remove from oven, stir, and bake uncovered for another 20–30 minutes.
Check orzo for doneness. If needed, add 2 tablespoons broth, cover, and bake 10–15 more minutes.
Stir until feta becomes creamy and coats the pasta. Garnish with fresh basil and serve.