Preheat the oven to 375ºF and spray a 9x13-inch baking dish with oil. Set aside.
Add the cherry tomatoes, sun-dried tomatoes, spinach, onion, olives, pasta, ½ teaspoon salt, and ½ teaspoon dried oregano to the casserole dish and mix to combine.
Separately, season the chicken with the remaining salt and oregano. Toss to coat.
Add the chicken and broth to the casserole dish and stir to combine.
Make a small hole in the center of all of the ingredients and place the block of feta in the middle. Cover and bake for 30 minutes.
Remove the casserole from the oven, stir, and bake uncovered for another 20 minutes.
Remove the casserole from the oven and check that the orzo is fully cooked. If the pasta is not fully cooked, remove the casserole from the oven, add 2 tablespoons of chicken broth, and cover for an additional 10-15 minutes.
Stir the casserole together until the feta has turned into a creamy sauce. Top with fresh basil.