Preheat the oven to 375°F (190°C).
Prepare the filling by combining spinach, feta, garlic, breadcrumbs, oregano, salt, and pepper in a bowl.
Using a sharp knife, create pockets in the chicken breasts for the filling.
Stuff each chicken breast with the filling mixture, pressing gently to secure.
Heat olive oil in a large skillet over medium-high heat.
Sear the stuffed chicken for 4-5 minutes on each side until golden brown.
Transfer the seared chicken to a baking dish and bake in the preheated oven for 20-25 minutes.
Let the stuffed chicken rest for 5 minutes before slicing.